Earlier quoted context omitted.
The incompatibilities between Schemes is one for starters.
The core Scheme language seems pretty small compared to CL so compatibility might not be the best argument to make. The best you can say about CL is that it was designed with the expectation of incompatibility. For libraries, CL has the same problems as Scheme so it's a wash. I hit this keenly when interacting with the OS (POSIX calls, mostly).
You doing work on Linux and need a very fast compiler? Use SBCL, 3rd party libraries continue to work (unless they're platform specific)
Doing work on OSX or Windows? Use CCL. 3rd party libraries continue to work (unless they're platform specific).
This flexibility is absent from the Scheme world, with the myriad slightly incompatible implementations and extremely fragmented library space. The emergence of Quicklisp for Common Lisp was so important that it alone KILLED Scheme for any sort of practical use. For most cases, one has not the slightest incentive to stray from using Common Lisp these days, and if he does, he better have a good reason.
What is the equivalent for Scheme? The SRFI hell? _shudder_