Because they are the surviving commercial vendors from the Lisp Machine days, with a complete graphical developer experience and optimizing compilers.
If you want a full blown Common Lisp experience, with a GUI based REPL, GUI debugger with edit-and-continue, and all the nice stuff you see from Xerox PARC, TI and Genera demos, they are the ones to get.
CCL has its own IDE which I find preferable to those of LW and Franz. It doesn't have the GUI construction set that MCL had, and unfortunately the IDE is Mac-only, but I find it much more productive than Emacs/Slime.
SBCL has an exceptionally good optimizing compiler now. It probably generates faster code than LW and Franz in most cases.
The CCL IDE is indeed good. I played around with CCL’s Cocoa bindings a lot earlier this year, then decided to, as another commentator said, throw some money at the problem and bought a LispWorks license to get CAPI.
