Why is it hard for guys to commit?

oh wow! I've heard of incidents too where the girl will be in a relationship with a guy for X amount of years and then get married to someone else for whatever reasons. And then those guys become Devdas. But here I'm talking about girls actually being serious, having honest intentions, being committed, but in the end the guy turns out to be not as serious. I've noticed this as well, the girls who don't show much interest in their bfs, get the serious/committed type of guys, and those girls who are serious, get not so serious guys...why is it like that?!