And languages become popular and unpopular for a reason.
For common lisp, those reasons are:
It's ancient.
Parties interested in it can't agree on anything so its development is stalled.
It claims to have a huge library which is tiny by 2009 standards, and doesn't have vital things like network i/o or unicode. Yes, implementations support those proprietarely, yet noone cares, because it's not in standard libs.
It has problems with reflection, which is doubly awful for a lisp.
CLOS is interesting, but for most uses smalltalkingly simple OO would be much more desirable.
Tool support is neliglible.