For a lot of reasons.

I think social control is a big one -- why do school kids get so mad at 'the weird kid'? You know, save for when that 'weird kid' is offensive in some tangible (albeit usually not PC way): they smell, they interrupt others, they're sticky, whatever--it's usually a control thing. There is absolutely no reason to ostracize a fourteen year old boy who, say, is more into manga than he is hip-hop. It's not so much that the manga (or chess or whatever) is offensive as it is it confuses the more alpha-kids -- they can't relate with the more "off beat" interests, so the only way they can regain that control is by being mean. Same applies for fat people.

Another reason is insecurity. You see flaws you perceive in yourself in other people, you're more inclined to judge them. I am a "woman of size," to put it politely, and I get really judgmental around other fat people. It's because I hate my own fat body. I feel like I can't fix myself, so I take it out on other people.

A third big reason is misogyny. I once asked a high school bully why me being fat was a bad thing. I said, "Alright, I am fat, why is that bad?" and he didn't have an answer. It was bad because it meant that I wasn't desirable. No one wants to have sex with a fat chick. No one wants to have sex with a fat chick because they take up too much space, you can't throw them around, they're not small like women are "supposed" to be. The fact the matter is other people's health really isn't your business. So, you can make the argument of like, "Oh, well that girl's just unhealthy," but then what's the justification for ugly women, you know? There's no reason to degrade women you don't perceive as ~~worthy of your dick~*~, it's uncalled for and women-hating. I'm not saying the reverse (women making fun of men) is better. But I will say you're a lot more likely to see an ugly or otherwise conventionally unattractive guy with a conventionally attractive woman than vice versa.

Anyway, that's my two cents.