Common Lisp is unfortunately a merge of multiple competing Lisps from the time period, much like Unix (SysV and BSD stuff). It was probably reasoned that a messy, imperfect standard is better than no standard or a restricted standard that is useless without proprietary extensions and requires constant revisions by a standing committee. There are good reasons to deviate from it and come up with a cleaner Lisp, but people better learn all the lessons Common Lisp has to offer before they do so.