[justinlardinois]

I think out of all the comments so far this passes closest to the real reason. It's simple: if you're developing for the web, you almost certainly have to use Javascript. That reality corrals so many more people into Javascript than any other language.

[inimino]

That is certainly part of the reason, but other platforms enforce language choices and don't have the same level of churn. Two popular mobile platforms come to mind.

[justinlardinois]

You don't have to use Java on Android and you don't have to use Objective C or Swift on iOS. You're heavily corralled towards them, but there are still other options.