Most Helpful Girl
Because it's their own damn business for them to navigate through and process at their own pace.
I really don't understand why some guys seem to think they are entitled to a female's affections and attentions because they are 'nice'. 'Nice' isn't good enough. There has to be chemistry. There must be mental intrigue that makes her squeal at your texts. You must make her want to pounce right on you and rip off your clothes. There has to be something special about the connection. Otherwise, your love and appreciation is equal to that from a grandpa.
Not only that, but "assholes" don't get girls by being rude, disgusting, jerks who treat females like shit. They have issues and baggage of their own that has reduced their morals and their capability to be decent and have a good nature. You have no place or right to label and judge these "assholes and total douchebags" simply because they are much better at the art of romance and attraction than you are.
Most Helpful Guy
Usually because the assholes are narcissistic and don't give a shit, making them come off as very confident. Confidence in a man emits attraction, and by the time the woman realizes she's dating a prick, he's already scored and has her hooked on him with an emotional attraction.
Also, many nicer guys simply won't go up and ask a very nice and good looking girl out, because they figure they'll get rejected, so all that's left are the assholes.