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.