Not too many possible answers and none of them are good.
They had kids without thinking long term
Someone else was done with them
They are usually very fake, one thing in public and a completely different person at home
They have unrealistic expectations from a long term relationship
They are usually high maintenence and will tear thru a man's bank account and leave him broke in a relatively short time.
They are sometimes alcoholics
I think the biggest one is they are very easily manipulated, or manipulative.
most are dumb.
p.s. yep, no good reasons. Even the ones who escaped an abusive relationship were dumb enough to get into that relationship