There has been some research (sorry I can't find it) which suggests that cyclists wearing helmets are seen by drivers as more competent and so they drive closer and more accidents result. This is controversial. Others can't afford a helmet, don't like the way you look with half a melon on your head, or find them uncomfortable, It's also an extra thing to carry around when you reach your destination. I guess most non-wearers just don't think about safety - which is why they jump red lights, don't look at junctions, don't wear bright colours or use lights, and ride on pavements. Rant over. I almost always wear one, though I have been known to forget.
