Insecure men who are generally useless at everything, which is why they don't get girls. If a man is smart, confident, good-looking, ambitious, witty, AND nice, will he be called 'a nice guy'? No. He'll be called cute, or sexy, or any of a number of positive terms.
The 'nice guy' tag is left as a consolation prize for those useless bastards whose only redeeming quality is their niceness. These guys can't make women feel special (apart from being an emotional tampon), don't have the confidence or style to show that they're able and talented, and don't have any touch of charm or wit. In short, they can't really compete with other men because they're too hesitant and insecure.
Thus, they deliberately pander to attractive women whom they put on a pedestal, not out of genuine concern for women as friends, but because they secretly want some, and this is the only way in which they can really try and get some.
But women can smell this, so they tag them as nice guys: the perfect friends, and the worst of all possible lovers (serial killers aside).
Woman 1: That John, he's a nice guy and all, but...
Woman 1's friends all giggle in shared understanding.
October 11, 2006