Javascript is the king of compromises. It's supported by every browser on the planet so of course support is massive. It's not as if a front end web developer can choose to work on the web and not use Javascript in some form or another.
Python is popular largely based on the fact that it's so approachable. It is the BASIC/ VB of modern times for whatever that is worth. It does scale up to larger projects and is frequently used for big scale stuff, but I suspect the fact that it's so ubiquitous has more to do with the fact that it's also easy to pick up and for companies to find people with Python dev skills (or train them up).
