Because people grow and act like mature adults later and later. That's if they do at all. They grow up never being asked to work or make a sacrifice. Then they get married and the first time something is inconvenient or they're asked to sacrifice something they want, they lash out and have a toddler-level temper tantrum.
While immaturity and selfishness absolutely play a part in the inflated divorce rate, I seriously doubt that kids in earlier eras (who got married much younger) were wholly self sacrificing.
In honesty the inclusion of women in the workforce has probably been the largest cause of divorce. It's a lot easier to walk away from a marriage if you aren't dependent on men to keep you (and your kids) alive. People stay in situations they hate do to necessity, I think, more often than out of pure selflessness.
Another large factor, I believe, is the tendency for Americans to isolate themselves in nuclear families. Countries with lower divorce rates tend to have much broader and interactive extended families. I would bet the stress of raising kids with no other support network has ended more than a few marriages. (I think we've all heard a both sides of the classic bitter argument involving the stay-at-home mom that tries to hand over the kid(s) the second Dad gets home from work.) Plus lack of close extended family ties has probably caused the degradation of many conservative/religious values, which is part of the reason there's less of a stigma associated with divorce.
I believe that there's definitely an association between the elevated divorce, and the near universal acceptance of premarital sex.
In both situations we see the erosion of a conservative/religious-based stigma.
Since the stay-at-home mom is no longer the only end goal for women, there's probably much less fear of a woman losing her marketability so to speak. Plus career minded women will take longer to get married, which means skipping sex all together in her early 20s (I'm sure that's a completely reasonable expectation) if she isn't willing to settle for some strange with a little less implied commitment.
A lack of a strong family culture probably results in quite a few more wolves sneaking into the hen house, not to mention the disproportionate number of women who leave home for college (57% of all college students are female) so the hens kind of go to the wolf house.
Plus in the era gone by there was a lot more fear to it, I think. I assume in 1940 the main birth control method was pull and pray. I figure there's a lot less shot gun marriages for that reason too.
Bottom line it's really freaking complicated. Because we're talking about a change in behavior patterns of a 300 million person population, divining an answer to inflated divorce rates based on the assumption that everybody sucks now, but didn't suck before (the less believable portion of that equation) is a little ridiculous, and implies that 1) Every divorce has at least one villain. 2)Married people are better/more mature/less selfish than their divorced counterparts. This sweeping generalization is irresponsible.