My theory is that the women who go for bums are women who cannot attract their peers or equals or who cannot make a relationship with an equal/superior work.
It is hard to be a successful woman in North America - too many demands on her (friend, caregiver, lover, mother, doctor, homemaker, cook and oh yeah, be excellent at your job too). So the guys who work with a successful woman see her bad side, her ball-breaking do-everything perfectly side, and we get turned off by it. So.......we assume or see that a relationship with a high-powered woman means we have to contribute to the housework too, and the child rearing and all that. Fuck that!! We want women who take care of us. In many cases, the women we work with can't take care of us....so we avoid them and date beneath our status, so we are taken care of. In many cases, not all. Thousands and thousands of exceptions.
So what does such a woman do? A strong woman who can't make it work with a peer? Well, thousands upon thousands of them make it work anyhow, with a man of equal or greater status. Lots of guys can handle a wife who works in a demanding job and lots of guys are willing to make the sacrifices and efforts required to allow both him and her to have successful careers. That's the norm, frankly.
But some women can't make it work with a high-powered guy, so they give up and go for the guy who is beneath them, economically or socially, but who treats them like a woman. So they get the best of both worlds - freedom to be a ball-breaker at work and have a successful career, and the pleasure of a he-man at home, who treats her like a woman. They sacrifice a bit financially and socially by dating beneath them (its easier for a guy do "date down" and introduce a lower-status woman to his circle of friends, than for a woman to do the same), but if they're happy, good on them. Takes a village and all that.
Just a theory. I'm sure there are many other reasons for a woman to date down.
