Part of the reason may be that there is (usually) no screening for ages, so you get people from all parts of the age spectrum. As such, you get a lot of immature people making comments. Teenagers tend to be the worst for being trolls, making sarcastic comments, and just being general complaining jerks without much to back up what they say. This is actually the main reason why even though I'm an avid gamer, I can't stand MMO type of games... There again you get a lot of that selfish, snarky comments where it seems like they get a kick out of belittling others instead of being constructive.
Also I think that YouTube offers a forum to show off your abilities... Usually a person wouldn't go through the trouble of taping themselves doing something unless they felt that they actually had talent; esp. if they plan to show that video to the general public. So if someone does film themselves and it turns out that they aren't as good as they thought, the public tends to have more of an aggressive backlash. Sort of like trying to get back at the presenter for wasting their time. Like on American Idol where at the beginning they show people that are terrible singers; it is a way for the show to entertain viewers through ridicule at the cost of the person that may have never sang in public before and so doesn't realize their lack of ability.