Besides the total domination of the web programming space, which is of course aided by it being the only option:
1) Used by choice even on the server and application development (where it was never the only option, and wasn't even preferable/viable before)
2) Fast pace of language development
3) A thriving package ecosystem with millions of packages
4) Adopted by all major companies
5) Three best of class runtimes (v8, JavascriptCore, Tracemonkey (?)) by 3 major vendors, with performance that smokes any dynamic language that is not LuaJit
6) Increasingly adopted as an embedded scripting language in all kind of apps
7) With a viable gateway into both a native trans-language runtime (webassembly) and a typed version of the language (typescript).
>How are them generics?
They're doing great. It's not that type erasure is that big of a deal, and Java might even get it with Valhalla eventually anyway. It's not a "backwards compatibility prevents this" issue (which is our topic here), it's a "no time devoted to add it in yet" issue.
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).
Besides the total domination of the web programming space, which is of course aided by it being the only option:
1) Used by choice even on the server and application development (where it was never the only option, and wasn't even preferable/viable before)
2) Fast pace of language development
3) A thriving package ecosystem with millions of packages
4) Adopted by all major companies
5) Three best of class runtimes (v8, JavascriptCore, Tracemonkey (?)) by 3 major vendors, with performance that smokes any dynamic language that is not LuaJit
6) Increasingly adopted as an embedded scripting language in all kind of apps
7) With a viable gateway into both a native trans-language runtime (webassembly) and a typed version of the language (typescript).
>How are them generics?
They're doing great. It's not that type erasure is that big of a deal, and Java might even get it with Valhalla eventually anyway. It's not a "backwards compatibility prevents this" issue (which is our topic here), it's a "no time devoted to add it in yet" issue.