Economic reasons. When people are living in virtual poverty, there is a strong economic disincentive to marriage for those on the edge of poverty, which is increasingly common in this economy. This disincentive becomes so strong as to become nearly absolute if either one of the partners relies on government aid of any sort and the spouse isn’t rich (as the spouses’ income is inversely correlated to the amount of aid the other spouse receives: say, that one spouse gets $15k a year in benefits, cash and material, and the other spouse makes $20k working: if these people are unmarried, they have a total income of $35k: when they become married, the benefits are dropped, based on the spousal income,* as it would be if the individual was working alone, for the total income of $20k).
*This is essentially the end result, but it’s accomplished through opening benefits to taxation.
I know couples in that situation, which precludes both economic welfare and marriage (one can have one or the other), and seems to be a real reason to me, the loss of 50% of income: only if the economic disincentives were removed, could I see if there was another reason, or another rationalization, that they would come up with to avoid marriage. Too many as well also view marriage as merely a “formalization of going steady”, and something that can and will be reversed and undone as soon as the “boyfriend-girlfriend” relationship of fornication would be broken off, whether formalized or not.
I’m sure there are other reasons, cultural, social, moral, that have undergone a general decline, so that people can rationalize away not getting married, but even for those who are faithful and true, the economic reasons remain, and remain valid, if the option is between destitution and/or food insecurity and marriage, and even if not between destitution, between a large cut in income and marriage (which, I imagine, comes down to cultural and moral problems, of people not willing to accept a lower standard of living than they possess, which also ties in to the restriction of “libertarian freedom” that scares some off of marriage).
I think part of the reason is the culture’s warped view towards procreation, and also towards the scare of what can go down economically in a divorce. It’s definitely a multifaceted problem, and, sadly, I do not believe it can be corrected.