In a classic agrarian economy, having a spouse and making children works out -- more children = more hands = more work done = more food or more product to market.
In a modern "pure" capitalist industrial/service economy, the economic model doesn't pay for the cost of the reproduction of the labour it depends on. So not a surprise when people do the same cost-benefit analysis for themselves and choose (or are forced) to have no children, and to not bother with marriage as a result.
The no-children "choice" often also ends up with no partner -- married or not. I personally knew several women in their late 30s and 40s who ended up leaving partners or leaving the dating scene because the men they were finding were not interested in or capable of being fathers. On an aggregate scale this adds up to a phenomenon.
Some advanced capitalist nations have gotten around this through high immigration rates. Others are attempting to get around it with debt to finance social programs or stimulus etc. In both cases it's really just "shooting the puck down the ice" so to speak. The next generation or the one after has to deal with the problem.
Marriage collapses two separate legal entities into a single entity, putting you even more at the mercy of the increasingly extractive financial/legal system. In a committed relationship, how does it make sense to throw away the flexibility of having independent legal entities for each of you, especially when creating new solid legal entities is quite expensive ?
With everything being increasingly financialized (eg the medical billing industry cancer), there are many sources of extreme liability these days. Long tail events that can stick you with some insane amount of debt that will drastically alter the course of your life. If we're married, any debts pinned on my partner automatically end up being my responsibility, meaning our entire life is completely at risk.
Whereas if we're not married, then each one of us is effectively holding half of our wealth in an informal trust for both of us. And for solidifying and committing to that arrangement, it would seem these trusts could be formalized in a way the courts would have to respect. Rather than defaulting to the expected traditional legal transaction, becoming a singular entity, and then having to grasp at sketchy asset protection methods and the like.