A few reasons:
They fall in love with the guy's "potential". Through either deception, half truths, or just outright lies, the guy may have convinced her that he is capable of so much more, if only he had the right girl to motivate him.
They think they can "fix" him or "be that really awesome girlfriend that he hasn't ever had". She recognizes his flaws, but instead of blaming these flaws on the guy (which is where the blame belongs), she blames it on those around him. "It's his mother's/father's fault he's this way!" or "His ex was a terrible person, and she made him like this".
She hasn't yet learned the lesson that "you cannot fix an adult". Smart girls are incredibly self motivated. They want something, they work for it. They think that losermen can be motivated. They can't be motivated for a woman. Or if they are, it's not the type of motivation that lasts. It's only temporary. "Oh, of course I'll stop drinking for you!" *three days later* "This beer is my reward."