I don't know CL, but from the subset of scheme that i know, i cant imagine how can this language fail. It just feels too good and the code is so beautiful. I could say for certain that scheme has made me into a better programmer.
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.
As for multiple implementations: CPython, Jython, IronPython, Stackless Python, PyPy. What editor do you use? Gnu Emacs with Slime; or, some of the commercial Lisp implementations come with IDE's; and there's work being done ("curl") to make Eclipse work well with Common Lisp. Java has several IDE's, such as IntelliJ and Eclipse. For Python, what I'm told by the Python experts I know is that they just don't use any IDE.
But much of what you're saying above is very much to the point. The comments on my blog posting go into these issues more.
Agreed! I like the Eggs repository for Chicken (http://chicken.wiki.br/Eggs%20Unlimited%203), but individual Scheme implementations just aren't compatible enough for such things to really benefit them all -- I'm pretty sure the lack of a module system in the core for R5RS is to blame. There's one in R6RS, but I hope it's not too little, too late.