Because today people don't think they have a duty to stay with one partner for a lifetime. In the past there were no divorces and women were not allowed to leave the family or they would lose their children. Today people are more independent, especially women and don't believe they have to sacrifice everything to keep a relationship going.
The other thing is that nowadays we are probably not trying that hard to make the relationship survive. There is much to be done once you are with somebody and it takes a lot of hard work to be a family/couple.