That is the question. It never used to be the question. It used to be 'When to marry?' not 'why to marry'. For years women were expected and expected themselves to find a husband young, marry and set up home with children and a pet, and dedicate their live sto running a good household. Perhaps because women have been liberated from what were once their traditional roles, that marriage is no longer a top prority and certainly not the only route to go down. In fact many people- both men and women- positively hate the idea of marriage, and can't imagine looking into a loved one's eye and promising whole-heartedly to spend the rest of their lives with them until death do they part.
In many ways it's a shame, because weddings are such joyous occassions (or can be) and they represent a couples bond and attachment to one another. But, unhappy marriages can be devastating, financially and emotionally ruinous, and the most isolating things in the world. Though being in a unhappy relationship is also incredibly isolating, somehow with the marriage label, it's even worse. The outside world expects more of a couple when they are officially married and have supposedly spent so much money on their wedding day. They are expected to be happy or to at least try their darndest to be so. As a result of this perceived outside pressure, many men and women feel they have to put on a front and hide their true feelings. The reason why there were more marriages or why marriages lasted a few generations ago is because couples had no choice but to keep ploughing on with their relationship. This is good in some respects, but in others, it is very destructive. Yes there were more marriages, but there were also many more desperately unhappy ones.