Because many do it just for attention. They feel a need to broadcast to people they are different from the majority. They also try to label them self's by wearing vintage T-shirts and wearing those thick framed glasses. If you don't do any of that then you're not a hipster. If you are there is nothing wrong with it, heck I wouldn't mind being a hipster, seems cool what they wear!
I'm vegetarian, the majority eat meat. I like many bands and artists people will never have heard of or listen to. Probably because the music charts are so corrupt with whatever plays on the X-Factor.
But I feel no need to make people know this about me. I personally have no problem with hipsters (they like the same things as me! :3) but many people just take a dislike to them because of their nature.