Python is my absolute favorite language without a doubt.
But Python is dying for ME
I think there is a lot of value in having front and back end code in the same language.
Python is significantly slower than JavaScript and I just don't want to have to run 2x the number of servers to handle the same number of requests.
I host my games on App Engine. Google has no public plans to move Python app engine to Py3. Python 2 EOL is just 5 years away. I move slow - need to start moving now.
-- Also while on the topic, let me just rant about how great the Python Standard Library is and how much I hate NPM and the javaScript ecosystem. I really hate having to evaluate 5 ways of doing every little thing. I wish the node folks would start building a batteries included standard library.