The point in hating the brony fandom is the same as hating anything else. They probably simply disagree with the fact that grown men are watching a show intended for little girls. I can see the sense in that, I truly can. I'm not anti brony nor am I a pro brony. I like to stand on neutral ground. I watch the show, I like the community. I don't do much else with the fandom in regards to fanfics, music content and whatever else.
The main reason is probably just to annoy those that react to it. They want a reaction, the moment they get one they have won. Don't bat an eye and just continue what you're doing. They just target bronys because, put simply, they are an easy target. Whenever I see someone say anything close to negative about the show. Everyone in the fandom comes flocking in arguing with them. Easy game is easy game.