That's a fair question to ask and one I struggled with at times.
It would seem that good guys are safe guys. Many girls, women, etc. are always claiming to be on the lookout for good guys, then they find one, date one, and break-up with one just as you described. For me, it seems like uncertainty, "Is this the guy I really want to be with? He's nice and sweet, but is that too boring? Where's the excitement? Am I ready to be with just him?"
A general rule, as women get older, you'll notice that their tastes can shift drastically. Some who used to like the bad boys are now looking for a bad boy appearance but a good boy heart. They start to realize they are ready to be with the next really good guy who fits them well. Obviously every person reaches that point at varying times and men have their own version of this, so we're not exempt.
The best I can say is keep on the lookout for a good girl and one day, you'll find her.