To some extent, many women are attracted to assertive, confident, dominant men. That doesn't mean they have to be assholes.
The problem with "nice guys" ("nice guys" referring to this group, as genuine nice guys are much more deserving of the term and usually have no trouble getting women) is that they often bring nothing else to the table. Yes, they are nice, but that is a pretty basic requirement for not being a terrible human being. If you're not nice then you're some degree of dick. In addition to being nice, you have to be somehow charming, have a talent, have something interesting and special about you.
The contemporary Internet "nice guy" movement, the ones who get "Friendzoned" (how dare she decide who she does and doesn't have sex with! You gave her a ride to that concert!), also feel entitled to affection and sex just for being decent, friendly, and fail to understand that no woman owes them anything, unless there was an explicit agreement that in return for acting like a friend, you get sex. These aren't "nice guys" but misguided, entitled jerks.
It's not that women don't want nice guys, it's that women don't want just nice guys. They want nice guys with a bit of balls and adventurous spirit, or something. But "nice" is not a merit on its own for a romantic relationship.
Back me up on this, ladies?