Why are people so obsessed with My Little Pony?

Every single thing on Earth has its fans and some of the fans just NEED to take it one step too far if you ask me.

But I guess that their reasons are not too different from let's say...christianity.

They do it because they found there some symbolism even if it works only for them, some kind of inner peace, they found guidance, friends or long-forgotten values thanks to the show. Some are using it to reflect on and improve their lives. And so on and so on.

Yeah, the challenged pony is, or rather was really there. In the pilot episode, there was unnamed background pony with crossed eyes. Just an animation error. But some fans started to spread it and even named her Derpy. And well... In one episode, she was given speaking role and her name was really Derpy, so the fans went completely haywire about how they established their very own character in the show.

I think it would not be far off to say that for osme of them "Derpy" was like their own daughter after that, but then some people started to threaten Hasbro with lawsuits, because apparently the only right way to treat handicapped is to erase all evidence of their existence and so Derpy was censored. They fixed her eyes, changed her voice and cut out her name.

That was probably the point you are talking about. Fans were really pissed off, started extremely aggresive campaign "save Derpy" and some are indeed showing signs of religious zeal. Hard to say if they really mean it as seriously as it sounds or they just want the publicity albeit negative one to point out only god knows what about that whole incident.

People these days are going through a strange phase where they like stupid stuff like that, pretty much everyone is into "STUPID" these days. Its trending pretty quickly.