MCL was a commercial product (for a time even owned, developed by and sold by Apple directly) with an employed team working on it. Apple had a bunch of projects and tools using it. MCL had a manual, a GUI, support, etc.

Clozure CL is the result of a project extracting/porting the bare CL implementation from MCL. It was open sourced. It got a GUI later, but never again had the polish and appeal of a product like Macintosh Common Lisp.