1) Huge institutional support. See: C#, Java, Go.
2) Give it 20 years. See: Python, Haskell.
3) Weird, idiosyncratic factors. See: Javascript, mostly.
So it's hard for me to get excited about these sorts of things.
Edit: fair point, Javascript is really a case of #1, with Netscape as the institution.
Writing traditional compilers and interpreters, or emitting C, or targeting LLVM have all proven to be good ways of getting production-grade programming language implementations created and usable quickly. Messing around with Parrot has never resulted in anything useful.
pg said that for a language to become popular, it has to be the scripting language of something popular. That's the category where the three languages I mentioned are falling.
So the answer is simple: one has to build something really nice in Nimrod and use Nimrod as the extension language of that thing. Maybe the embedded world would be a nice field from Nimrod due to having nice interop with C? (I haven't used Nimrod in my life yet, although I am tempted because it looks very easy).
Ruby I guess is a (3): Rails is a killer app.
C, well, it's the a major accomplishment of human civilization and has held up for half a century. I'd call that idiosyncratic.
As UNIX spread into the industry, so did C.
All languages required by OS vendors as the official ones to target their OS, succeed in the market if the OS succeeds.