Common Lisp and (especially!) Scheme are really elegant languages, sure, but there's really something to be said for being able to install the one clear Python implementation* and getting a huge standard library with lots of documentation and a large community, upfront, versus trying to figure out whether you should set up SBCL or CLISP or PLT or Chicken or ..., then finding there are several libraries for what you're trying to do, but you're not sure they're compatible with the compiler/interpreter you're using, etc. And then, what editor do you use? ("What's this SLIME thing about?") And this, that, and the other thing. That is a lot to figure out, all at once and upfront, before you can try it out. Making it really hard for curious people to play around with Lisp means that few people will see its potential.

All of the above could just as well apply to Ruby or even Haskell. I think Python, in particular, has been really carefully designed to give a good first impression, though. I don't know if that was deliberate or not, but it's brilliant. (I know some people balk at the indentation-based syntax, but I bet many people coming from e.g. a Perl or C++ background think, "Whoa, this looks really clean." "Executable pseudocode.") Lisp doesn't really seem to focus on making a good first impression. (On the other hand, OCaml just says, "Hello!" and immediately backhands you. It makes up for it once you understand the type system, but I'm not at all surprised the OCaml community is small.)

Stuff like Lisp in a Box helps a lot, but it wouldn't hurt to have a link to it in 300-pixel-tall burning letters at the top of lisp.org, you know? You can't even go directly to a download page for any implementation from lisp.org. (When you click the download link at the top of Haskell.org, it takes you right to GHC's download page.)

I've also frequently heard the Lisp community can be rude or off-putting, but I haven't experienced this personally. I could see them being a bit gruff and frustrated about answering a lot of the same questions over and over again, though, and seeing languages like Python and Ruby getting so much more attention than Lisp can be heartbreaking.

So, basically: Marketing.

* I know about Stackless, etc., but which does python.org point you to? QED.