This is really quite simple. "Upward Mobility" This means finding a man with money (usaully family money) to improve their own prospects and finding a father for their kids who will provide them the right genes (those providing good looks).

With the invention of the pill and change in sexual morays in the 60's, the era started when women could actually influence this. Very quickly, most women became "players" by using their sexuality and other favours to attract men. Their targets were the guys who were 9 or 10 on a scale of 0 to 10 but women from 4 to 9 could easily play the game. (10s didn't need to play and were usually the eventual choice of the men anyway) This resulted in a lot of competition for such men who knew that they were in demand and therefore refused to commit to any particular relationship but were experts at stringing along many women who were eager to believe them.

Meanwhile, guys who were merely 7 or 8 would tend to be set up with the women who dropped out from this scene. However, the guys of 6 or less had few options and despite being great marriagable material, were often single. I know many like that.

Eventually, many women in the 4 to 6 range realized that they had no hope of nailing down "Mr. Right". However, they were unwilling to lower their sights and settled for getting the genes - but not the father - for their kids. Then they had some hope that some of these single guys would step up to the plate. As the decades have passed, fewer and fewer are doing this, since marriage is no longer expected of men.