In the programming-language-as-a-spec days, people versed in programming language design created the dominant programming languages. In the programming-language-as-an-open-source-project world, languages tend to be created by people that are relatively clueless about language design at the start (this holds for at least Perl, Python and Ruby). I don't know what the root of such is, but there seems to be some negative correlation between the design integrity of a given language and its popularity (for web hackers at least – Java and C# adhere to the old model).
In contrast, Lisp seems to be fairly popular among folks that are programming languages enthusiasts. So for whatever reason, there's a gap between the folks that are creating new popular languages, and the sort of people that tend to be Lisp fans.
The other bit is that, well, Lisp looks funny. This is something like someone looking at a language that uses Greek or Cyrillic or Arabic alphabets and assuming that they're by default harder languages because they use different characters. In fact, a reasonably clever person can learn a new alphabet in a day. It's a totally artificial barrier for someone who's serious about learning the language since the investment to learn the new alphabet is dwarfed by the work of actually learning the language, but it tends to make it scarier if upon casual inspection. I suspect Lisp pushes some people away via a similar artificial barrier.