Why does widgetsnbextension 4 officially support only Python>=3.7?

I have two environments:

• IPykernel environment with ipywidgets 8.1.5 installed
• Jupyter Notebook 6.0.3 environment (Python 3.6)

According to the installation guide I need to install widgetsnbextension in my Notebook environment. But I have to specify pip install --ignore-requires-python to install it there. There's almost no Python code in that package, why it doesn't officially support older Python version?