Because society hates anyone who's a bit more diffent than the homogenous herd they are. They will try to bring them down however they want. You like ponies, well society thinks your a homoloveual neckbeard who spends their entire life jacking off to pictures of colorful ponies. Is that stereotyping, yes it is, is it wrong, yes it is. If you even question one thing, get diagnosed with one thing, society immedieatly slaps the stigma on. That's how it works and I've gotten used to it. It's a way of life now.
stop pretending you're being persecuted for being a brony when it isn't even relevant
also, the easiest way to stop being accused of being a horseforgeter neckbeard is to stop associating yourself with a group known for just that