Because the people representing drug users to the public as a whole are, well, average drug users. And the majority of drug users are messed up in some way or another. I don't believe drugs did that to them, I believe they were pretty much like that all along, it's just that those kind of people are attracted to drugs for some reason. However, when looking at legalizing it, the average person looks at the guys shooting people for their drugs or money to buy drugs and draws an incorrect but understandable conclusion about it.
It's like how the average American's experience with Islam comes from terrorists. Not representitive of the religion specifically, but for some reason it seems to attract the wackos.
If the people supporting the legalization of weed didn't wear hemp pants, live with their parents and do absolutely nothing productive, then maybe we'd see some changes. But until you see regular members of socciety advocating drug legalization, all we have to look at are these really poor excuses for members of society, and I don't think that makes anyone want to legalize drugs. Not me personally, but I understand where other people are coming from.