Profusion of dialects - lack of libraries. There is an inverse relationship between the success of languages and their power. C is cumbersome to write; therefore, everyone treasures the libraries that exist and takes pains to remain compatible. Java, even assuming that it made it 'halfway to Lisp', is clearly still cumbersome enough. Python is already too powerful, but it benefits from its slowness. In many problem domains (GUI, databases, numeric computing), it is way too slow, forcing you to use C-coded extensions, which again help to prevent fracturing of the user base. Where it's fast enough, you get a bazillion of half-assed libraries and frameworks. E.g. Python web programming is a mess and, compared to PHP and Java, not popular at all. (There are also limitations to the VM that matter here but I'm not going into those.)
Lisp is so powerful that it lets everyone modify it, and so everyone goes and creates their own dialect of it. It compiles to fast, native code, so no need to adopt and maintain libraries shared by a large user base.
There is a nice language called D. Unfortunately, it comes with two incompatible standard libraries. How do you like that as a new user? Well, in Lisp it's the same, only times 257. Throw in the oatmeal cum nail-clippings visual appeal (L.Wall) and you might well wonder why it is not less popular than it currently is.
