I think the dynamics of dating are such that women can get men that are higher quality if they don't want commitment. But if they want commitment, then they have to lower their expectations in general. Some women refuse to lower their standards to get the commitment. Others lower their standards to get the commitment, but then are left wondering "what if they hadn't settled?"

Another thing is that a lot of people overestimate how good they are and who they can attract. It seems like people don't like to think about it. So maybe by settling they are actually just coming to the realization that they aren't as good as they thought. (Whatever the market will bear.) Part of the problem is it's not polite to tell other people what's wrong with them, so they never get any feedback, hence they don't have a good estimate of their true worth.