Because despite the enthusiasm of the advocates, it's not actually the most useful programming language. "But I used it for X and it was insanely productive!"

Because despite the enthusiasm of the advocates, it's not actually the most useful programming language. "But I used it for X and it was insanely productive!" Great, but there's a lot of programming that is not X programming. "It's just that the rest of you are not enlightened!" Right... the maharishi's people tell me the same thing. "It perfectly fits the way your mind works!" It perfectly fits the way your mind works. It doesn't perfectly fit the way most of our minds work. Lisp winds up not being the most productive language for the vast majority of programmers. So it's not very widely used.