What would python add that Javascript doesn't already let you do? Granted it can make programming easier in my opinion, but think of the complexity it would add. Any major website would need to either have two versions of their code (one JS, one Python), or more likely just use Javascript (you can't turn away visitors because their browser doesn't support your favorite language)

The answer is that we need a VM supporting many scripting languages.

I recommend the demo where some guy is scripting in Ruby.net and following on Python... The sites can be implemented in any scripting language.

That doesn't even begin to handle the backwards compatibly argument. We still have people using browsers like IE6, so for a long period of time you would need to write code in both languages.