I think it's to do with the fact that divorce is more socially accepted these days.
Men have become less chivalrous, women have become less docile.
In the olden-days, the man would rake-in the money, come home to his wife who has made dinner, cleaned the house and raised the kids.
These days, women want more power- they want a say... but it still isn't entirely socially acceptable for a guy to be a 'stay at home dad'. Why?? Isn't this equally as sexist as not allowing a woman to work??
When a man and a woman both come home from being stressed, leave their kids with nannies and babysitters, dont put aside family time, get married too young, get married out of pressure, get married when they're unsure... it can all lead to divorce.
Society has also become more sexual... things like hardcore porn and 'swinging' weren't so common before... and it was often accepted for a man to cheat on his wife, but make it sound more innocent by calling the other woman his 'mistress'.
Relationships have always been dysfunctional... but it was harder to get a divorce without the whole town going into shock/gossip mode.
People don't feel the need to commit to making their marriages work anymore, when the easier way is just to run away from it.