Because Khadafi (old accepted spelling) was a cowed, defeated enemy who posed no threat to us or our interests anymore, and because of who it was who was attempting to (and who, with our help and overt support, suceeded), the Muslim Brotherhood. This is similar to what's happening in Syria now. Don't make the mistake of seeing everything in pure black and white. Sometimes your choice isn't between a "good guy" and a "bad guy" but between a "bad guy" and an "even worse guy."
Right now, Obama is supporting pro Caliphate force at every turn, over independent dictators who are unwilling to turn over their nations to the new, unified pan-Muslim superpower which is currently being created.