Eh, on the serious side, despite metal culture being typically open and friendly (outside of the pit, in the pit you are hamburger to be chewed), every group has assholes.
There are people that just like feeling superior, so anything they aren't into sucks, and they'll make up the criteria start determines what does and doesn't suck so that their opinion is validated.
That's all it is. You see it with bookheads, with hip-hop heads, pop freaks, whatever.