Say we are talking about players. Players approach women, know how to talk to them and make them feel good about themselves. No woman thinks: "he has lots of girls, let me date him"... players are great at disguising their womanizing ways (in the beginning). Nice guys tend to be shyer, it's not that women don't want these guys, it's just that women have been conditioned not to make the first move - and the guys just don't create chance for themselves, I don't blame them, it's horrendous putting yourself up for rejection like that - but it's equally pointless to blame women.
Also.. put it this way... you're a nice guy now... but if you were super confident and all the girls wanted you - would you resist it on principle? Would you shrug them off for one woman?
The true test of a nice guy is a guy who can get the girls and chooses to be faithful. And these type of guys don't have to wonder why assholes get girls.