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The roots go back to Feminism, which pushed for much more liberal divorce laws (among other things). Few here realize that in the 1950s and before, it was extremely difficult to get a divorce - judges only granted them in cases with exceptional evidence, and often only with church consent. And being divorced came with a big social stigma, of the type that you'd only get today if you were a known pedophile or something. Even a p*rn scandal isn't nearly as big a scandal today as a divorce was before the mid-60s.
Thus, getting married in the first place was taken MUCH more seriously in terms of compatibility and the expectation to work together and find solutions, than it is today. Now, people's attitude is: if I get married, and it doesn't work out, I'll just get a divorce, so why care much if we are really compatible?
There are other contributing factors, but that is by far the largest.