The Lisp Curse
Occam's Razor suggests that the reason there is no dialect of Lisp as popular as less expressive languages is that no one happens to have created one yet.  What languages are has changed.  Languages used to be specs.  Now they're open source projects.  Open source projects seem to succeed based more on the energy of their initial organizers than anything else.  There are only a few big language-project organizers, and none of them happens to have chosen to implement a language that is a dialect of Lisp.  That's the sort of thing that's true until it's false.  E.g. Clojure could be the counterexample.<p><i>Maybe</i> there's more to it than a small sample size, but that seems the likeliest explanation.  The second most likely explanation is almost as mundane: that the reason new dialects of Lisp have trouble attracting adherents is that mainstream programmers are put off by s-expressions.