I'm not a CL expert, but the existence of multiple implementations of CL doesn't appear to have caused much fragmentation. The language did evolve beyond the standard (e.g. threading and the meta-object protocol), but if I'm not mistaken all popular implementations of CL (SBCL, Clozure CL, Allegro, LispWorks, CLISP, CMUCL) implement the full meta-object protocol, and even if their thread implementations use different libraries, isn't that pretty common among programming languages that aren't considered fragmented? Different implementations of C are certainly like that. Unless the OP really meant to ask why some languages have one main implementation and others have multiple, but that seems like a very different question (whose answer in the case of CL is obviously not that it broke off into multiple languages but rather that it was designed as a standard without any official implementation to begin with).