Just to break what could form into an endless circlejerk...
I'll tell you why. It's because of the morons you see all over the internet posting My Little Pony things where;
A) it is completely irrelavent
B ) they are force feeding non-bronies their bullshit, trying to 'convert' them
C) they are trying to purposefully stand out to seem different, and thus recognised.
I'm not saying that all bronies do it. I'm not even saying most of them do. I'm saying that because there are unhealthily obsessed idiots cramming their My Little Pony down people throats in places like YouTube comments, Tumblr responses and non-MLP forums or boards, there will be people that make the generalization that ALL bronies do. And don't you dare tell me it's all trolls acting in a guise
Just as some women think that all men think about is sex, or some men that think women belong in the kitchen, some people will think all bronies are annoying, incessant maniacs that serve to be nothing more than a nuisance.