I know you're a troll, or at least you like to troll, but I'll try to answer honestly.
I think there are a couple of reasons. First, bodybuilding/powerlifting isn't actually healthy for you. Sure, it's healthier in the basic sense that exercising in some way is better than sitting on your couch watching 10 hours of TV, but in terms of exercise, extreme lifting is pretty bad for your heart, bad for your back, knees, etc. Grandmas don't powerlift to try to live longer and stave off asthma. Because the sport isn't centered around doing an astonishingly healthy activity, there's less of a stigma around doping, which is very bad for you.
Second, running is a less vain sport than bodybuilding. Sure, runners are vain, but the idea of the sport isn't to look good, it's to run fast. That's not the case with bodybuilding. Powerlifting is more like running, but serious powerlifters who compete are relatively rarer--and would probably frown a bit more on doping--than the majority of people who lift, who are more interested in looking good than anything else. If you want to look good, you aren't going to care about the purity of the sport as much. Hence doping.
Third, I think runners are generally more educated on the dangers of doping than powerlifters/bodybuilders. Maybe that's my bias. But runners tend to be more cerebral and intelligent than athletes in many other sports (perusers of this message board excepted). There's a chance powerlifters/bodybuilders don't realize doping puts them at risk for all sorts of health issues down the line.