More people have their own money and are able to live fairly well without being married. This gives them "options" that previous generations may not have had. The more options one has the less crap they are willing to take. As for those in second marriages they aren't likely to invest as much time on sinking ship the next go round. Lastly there is no stigma with regard to being divorced in our society.
Sometimes I wonder how many couples are "happy" out of the so-called 50% of people that remain married. I personally know of a few couples who are married on paper but haven't seen each other in years! They live apart and date other people. I also know of other couples living in the same house but in separate quarters and have almost nothing to do with one another. These are just some of the couples that are being counted in the 50% of "married" people. I call them "The Emotionally Divorced". :-)