For REs, Clozure CL was being used in part because faster compile times were more important and run time preformance was more than adequate; stateless business middleware is a very different beast than compute intensive route construction, where we can be sure the cutoff in optimizing choices is based on response time.

There were other reasons for Clozure CL, including the fact that it has a company behind it, one who's principles Dan and others have had long relationships with and ITA was buying one (man?)day a week of their services to support RES. SBCL is (has always been?) a volunteer effort.