Because once reports of the debris were relayed to the crews of the space stations, they would have evacuated.

"Houston, we have a problem" is a famous phrase from the 1970 Apollo 13 mission, and "Houston Blind" is a phrase that was used in the context of the movie "Gravity" (2013) meaning a communication blackout between the characters and the Mission Control Center in Houston. In the movie, it refers to the moment when the characters were out of communication with the Houston Mission Control Center, due to the failure of their communications systems, and the destruction of the space shuttle. They were effectively blind to the assistance and guidance that Houston could provide. Kowalski explains that they continue to communicate in the chance that Houston can still hear them even if they can't hear Houston.

While spacewalking outside of the space shuttle Explorer on a mission to repair the Hubble Telescope, two astronauts—medical engineer Dr Ryan Stone (Sandra Bullock) and veteran astronaut Matt Kowalski (George Clooney)—are suddenly bombarded with debris from an exploded Russian satellite, irreparably damaging the shuttle and station and leaving Stone and Kowalski adrift in space, tethered to nothing but each other.

Gravity was filmed from a screenplay written by Mexican filmmaker Alfonso Cuarón and his son Jonás.

The MMU (Manned Maneuvering Unit) was used by American astronauts during the mid-1980s, but it is not currently being used. Dialogue in the film indicates that Kowalski is using a fictional, new, presumably more technologically-advanced and fuel-efficient version of the MMU.

Sandra Bullock sat on a rig with a bicycle seat and had her right leg strapped into a two-part brace inside a specially-made chamber. She then mimed movements that were carefully choreographed and a camera rig was rotated slowly to create the illusion of her's and the ISS's rotation. Lights were also placed in strategic spots to capture the shining of the sun in the window. In post-production, Bullock's right leg and the braces were erased completely and her bare leg was recreated with CGI. In the special features of the DVD and Blu-ray disc editions of the movie there's a short documentary showing how this effect was created. Director Alfonso Cuaron himself says that the image was specifically designed this way to make Stone look like an in-utero child, hence her body's position and the positioning of the hoses in the background suggesting umbilical cords.

The man's name is Aningaaq, an Inuit fisherman in Greenland. Aningaaq happens to pick up Stone's transmission from space on his own radio pack. She finds him a comfort in the face of her situation. Director Alfonso Cuaron's son, Jonas, co-wrote and directed a short film that shows the conversation between Aningaaq and Stone from Aningaaq's perspective on the ground.

Stone would eventually have succumbed to the rapid loss of oxygen and temperature change if the door had stayed open. As can be seen earlier when she and Kowalski return to their ship, the crew inside has been frozen solid due to prolonged exposure to space temperature. Lots of experiments on Earth and in space and simulations with altitude chambers have given scientists a very good idea of what would happen if a person is suddenly exposed to space without a pressurized suit. As paradoxical as it seems, the body wouldn't freeze immediately, despite a temperature well below -200° Celsius (-328° Fahrenheit). The reason is that there is no gas in the space vacuum that can carry sound or absorb the heat; most body heat would disperse as radiation, which takes a while longer. So freezing to death within seconds as seen in Sunshine (2007) and Mission to Mars (2000) would not occur. The absence of pressure would cause the gases in our lungs and gastrointestinal tract to expand rapidly, which is why critical damage can be delayed by exhaling before the exposure. Normal air pressure on Earth keeps liquids such as water in liquid form; a vacuum causes the water in the skin and muscles to vaporize and expand, causing small superficial arteries to burst and several body parts to swell. However, the human skin is resilient and will be able to withstand the internal pressure build-up, at least when one is exposed from normal pressure to a vacuum (normally a drop in pressure of 1 atmosphere). The extreme bloating from Total Recall (1990), the excessive skin-tearing and hemorrhaging from Event Horizon (1997), or violent explosive decompression á la Outland (1981) would not happen either under such circumstances. Decompression with explosive effects is only known to occur with pressure drops of several atmospheres (as seen in Licence to Kill (1989)).

Another gas affected inside the body would be the considerable amount of nitrogen dissolved in the blood. The loss of pressure outside the body causes this gas to expand as well, and form bubbles inside the blood vessels and tissues. This occurs most often in the joints, and causes decompression sickness, also known as "the bends" or caisson disease, which can be painful but is rarely lethal. Thus, exposure to space vacuum does not cause an instant death due to all the blood vessels in the head bursting all at once, as was suggested in Mission to Mars. As unbelievable as it sounds, the first 10 seconds or so in space would be uncomfortable, but you could still manage to work with full mental capacity and attempt to take countermeasures; there is still enough oxygen in the blood to keep the brain working, as long as the blood remains circulating. After those 10 seconds, oxygen gradually stops entering the brain (hypoxia), the skin turns blue from oxygen deprivation (cyanosis), and the person would begin to lose consciousness. So the crew of the ship would have passed out long before they had felt the effects of the freezing. Convulsions would occur due to lack of oxygen in the brain, but the heart will continue to beat for a while. All negative effects up until that point are generally reversible. It is not until an estimated one-and-a-half minutes that the vacuum would start to affect systemic blood flow, the heart stops, and freezing becomes a serious problem. If a person gets back into a pressurized environment with plenty of oxygen within that time frame, resuscitation is a very likely outcome with only minor injuries.

So, the depiction of the opening of the door was quite realistic; had it really occurred, Stone would probably have lost consciousness after about 10 seconds; but within the shown time-frame of re-closing the door and rapid re-pressurization, she would have awoken quickly with no ill effects expected. Actual NASA space medicine documents confirm that an individual can/could survive up to 1 minute in a vacuum. This came up during the filming of 2001: A Space Odyssey (1968), when Dave re-enters the emergency airlock. (Source)

For the most part, the film does make an effort to follow the laws of physics as realistically as possible, even down to the depiction of no sound in space. However the film is not always scientifically accurate and that some liberties were taken in order to sustain the story. These include:

(1) According to NASA astronaut Mark Kelly, blowing up stuff in orbit makes a big mess, but it doesn't send a giant field of shrapnel hurtling at high velocity toward a spacecraft that is circulating Earth in an entirely different orbit. So the idea of debris traveling continuously around the world at a speed of 20,000 miles per hour (32,000 kilometers per hour) is implausible. Other sources point out that the magnitude and proximity of the debris fields relative to each other as seen in the movie is not realistic either. Finally, space debris could easily reach a speed of over 6 miles per second; a person in space would not be able to spot even the largest pieces of shrapnel moving that fast.

(2) According to NASA astronaut Mark Kelly, you... can't just point at things in space, head off in that direction and expect to get there. NASA now understands that by pointing at something and accelerating, you increase your altitude, slow down and instead move away. Today, we know that the best way to join up with another spacecraft is a slow procedure that takes an entire day in the space shuttle - too long for the supercharged momentum of the movie. 

(3) Stone's tears would not have formed free-floating tear spheres. The liquid's surface tension would make them cling to her skin or eyelashes. Furthermore, in zero G, her hair would also float around her head, whereas in the movie, it remains perfectly modeled.

(4) According to Zeb Scoville, NASA expert in spacewalks, the maneuvers Kowalski uses to rescue the free-floating Stone would have completely burned up all of his fuel. This is speculative on Scoville's part and is partly explained by dialogue in the film which states that the unit Kowalski was using was a new—presumably more fuel-efficient—version of real-world MMUs (Manned Maneuvering Units).

(5) Media astrophysicist Neil deGrasse Tyson pointed out that it would not be possible to spacewalk from the Hubble space telescope towards the International Space Station, as they are in completely different orbits. However, in the film's universe the HST, ISS and Tiangong seem to share the same orbit, making it more plausible.

(6) When Stone is free from the robot arm, she spins around about once every second; this later slows down to once every 5 seconds. This would be impossible, as there is no air resistance in space which would slow down the rotation. She could have gained equilibrium by spinning her arms in the direction of her rotation, much like a cat uses its tail to land on its feet. The space craft is also spinning after the impact that separated Stone from the craft, when they return the craft is stationary.

(7) During the spacewalks the astronauts never pull down their helmet's gold visors in order to shield themselves against the sun's unfiltered radiation. This would lead to severe sunburn within minutes. But then we would never see the actor's faces for a good portion of the film.

(8) Most notably, when Kowalski is tethered to Stone, he did not need to release himself. Once the initial momentum had been absorbed, no additional tension would have remained on the tether. She would have been able to retrieve him with only the slightest pull (he's weightless). Also, if he was released, he'd remain nearby, because there was no other force acting on him.

(Source)