Because all the social misfits congregate online. They don't fit into the real world. So they hide from it and lose themselves in the online world. In the online world they find other social misfits, and sort of feel at home. Not necessarily a bad thing, but not the best thing either. But they ain't gettin' any online. It doesn't fulfill what they really want and need. So they need someone to blame. If it weren't for the online world and internet they'd all be "out of sight out of mind". But now they are showing themselves and venting their frustrations. Because they can, without leaving the security of their homes.
OK, I just made all the shit up. But probably not too far off.
People on gag use gag to bash everything. Race, gender, religion, political views, age etc. It's the internet and people can get away with this stuff. Sadly they don't have the balls the actually say stuff like this to their face.