Because back in the day, people did this exact same shit with pokemon. And star wars. And Star Trek. And Power Puff Girls or whatever.

But the only place you knew to see that was on TV. Which meant someone had to create a television show, or a segment, devoted to showing you these things.

Now we have the internet. There is no need for production costs. You just get a camera on your fucking phone, go to where the shit is, take some pictures, and bam, you have a story. You have pictures. You have videos. Look at these weird motherfuckers.

Now, I won't argue that the amount of wackjobs attracted to MLP isn't larger than it was for previous television shows, but the reason it seems so out of control is that everything these days is so interconnected. Between facebook, twitter, reddit, and other social networks, the wackjobs can all find each other, reaffirm their wackiness, and 'regular' people can all stand on the sidelines and point and laugh.

You hear about it with the mlp fandom because it's perceived as a television show for little girls. So adult dudes watching it for any reason at all is enough for ridicule and enough for people to share it.

Did you know there are people just as obsessed with Steven Universe? Or Gravity Falls? Without someone shoving it in your face and going 'look at this weird motherfucker' you'd never know about it.

Case in point, think back to every weird brony you ever heard about. Now subtract all the ones you saw on reddit, or facebook, or some 4chan copypasta or anything like that. Subtract all the weird bronies you saw on the internet and you've got like two left (Unless you live near a con or somesuch). You'd have know way to know these motherfuckers existed.

That's exactly what it was like before the hyperconnectivity of social networks. These people existed, they just weren't a connected group feeding off the wackiness of eachother back then.