Because that is the online community that has amassed a reputation for taking social issues to their most severe (and often unreasonable) depths... whenever I come across a person that wants to make a non-issue into an issue (or make a legitimate issue into A SCREAMING BONFIRE OF DRAMATICS), they are usually spending a lot of time on tumblr getting these ideas that they can be policing and alienating to everyone they talk to.

It's kind of a shame, because so many of these issues are genuine problems that we need to be talking about, often and openly... but like pretty much all social movements a few radicalists have to drive it into the ground and turn it into a joke for most rational people.