I don't know that I'd agree that Common Lisp is "extremely" fragmented. It's true that there are probably more implementations than we really need, but they do serve different niches. Anyway, while "extremely fragmented" might have been a fair characterization a few years ago, there has been enough work done lately on portability of libraries -- much of it driven by Zach Bean's Quicklisp -- that switching implementations is often straightforward now.

CL is defined by a standard which most of the implementations work pretty hard to adhere to. Of course, most of them have their own extensions, but a lot of these (particularly in the important areas of multithreading and foreign function calling) are now abstracted by portable libraries. This is a very different situation from the Scheme world.