Brian Mastenbrook made a post questioning why Lisp is unpopular. To be quick, it's unpopular for these reasons:

• Sockets: Where's my standard socket library to talk with the rest of the world?
• GUI: Where's my GUI API that's on par with Trolltech's Qt, C#'s Forms, or Java's [whatever it's called, Swing?]?
• Threads: Not so important, but where's my standard thread library?
• FFI: A standard way of being able to interface with programs written in other languages would be nice too. There is UFFI but a standardized way would go farther.

I think those are the big four. I can pick and choose what XML parser I want to use. The above are required to be on par with the languages Brian mentioned.