Because they do not love themselves enough so they treat other people the same.Just keep in mind that they're not really mean:you're not just the one meant to get to their good sides.In fact,they feel much more pain than people like you because they still haven't found self-acceptance.:)
Because they're self aware and aren't into trying to eschew other groups of people, they know who they are and they're proud of it. No self loathing or what does it all mean stages of existence, they've already accepted who they are and if you're accepting of them they'll give that in return.
People under those classifications were once on the outer fringe of society, so they tend to be accepting of those who have alternative lifestyles. That's why they tend to not be so judgmental.
I think nerdlings and geeks are basically more self-conscious people, so they're easier to deal with than self-centered confident types. These tend to be popular -- for that exact reason -- confidence is attractive to women. Taking a nerdling for a date is higher risk, because your friends may give you a hard time for it. So it's easier to date jerks cause they're the more popular types. Plus they ask..... (I'm basically a nerdling at heart. LOL)