Who Killed Hannibal? - Why do girls always go for douchebags?

It depends really… Yes a lot of "nice guys" are fakes who play nice thinking they can hook women in by acting sensitive and caring. However in general a genuine "nice guy" is often overlooked since they more often then not remain passive, and try to build up a base so that they can show that they are kind and reliable and that they would make a solid long term relationship material. However since the build up takes a long time the more aggressive jerk types are more then willing to take the direct approach and can usually hook the girl much faster even if the relationship is guaranteed to be short and painful for her in the long run, but the up frontness and confidence acts as a strong first impression (or they're got 10/10 physical looks and can just use raw physical attractiveness to seduce).

TL;DR: "Nice guys" will more often take time to build up a relationship, Jerks are more likely to be direct right away.

That, or just being a dick/creep.