Maybe it is hideous to them since Bronies are Furries....according to that Furry Chart, Equestria Girls are only around 10%, while the normal ones are 70 or more %.
Being neither Brony nor Furry, but I'm currently doing some research in that direction, since I plan a story set in a multiverse and want to avoid making the non-human characters furrybait.
I figure teenage girls are better for marketing dolls and crap like that.
Was originally going to be more sarcastic "What if the reason Bronies hate Equestria Girls... is because Hasbro didn't make it for men but for little girls."