When C began to spread, it was significantly better than alternatives for writing operating systems and associated tools on commodity hardware. Part of that "better" was cost - cost of the machines it ran on, cost of buying a compiler, and cost of implementing a compiler. (The mistake the anti-C people make in saying "but other languages were so much better!" is that they forget that cost was part of "better".)
C became entrenched in Unix and, later, Windows. But it didn't start entrenched. It started by being better.
If Lisp were significantly better, with everything considered, it should have won by now. I mean, yes, there are a lot of people chasing the current hotness. But the programming profession is not made up entirely of fools and sheep. Better languages get noticed, get used, acquire momentum. (Note well, however, that "better" again is "better with all things considered" - it doesn't equate to "better" as defined by theorists who about how much better their language is, even though it's a little-used language.)
PHP can be considered a mainstream language today. Even COBOL in some parts of the industry. Are they any better because of that?