Well, consider, about 50 years ago almost every child was raised by married parents, today about half are.
Having so many people with no lived experience means lower cultural forces behind asking them to participate in an expensive party, even though marriage is so good for raising children (which used to be the purpose of marriage).
Why is an expensive party important? You can certainly get married at a Church or at a courthouse for pretty cheap and a party isn't required, but even if it is, you can throw a decent party without thousands of dollars spent. I've been to many of them in small towns across the Midwest.
If you are already cohabitating, marriage is an expense you can indefinitely procrastinate.
Especially if you see what others are doing on social media, your mental model of the expense continues to inflate and the pressure to procrastinate grows.
Historically marriage was seen not just as commitment between two people but between them and their community, and them and G-d. By committing to the marriage in a public, religiously-affiliated place, the couple is acknowledging their responsibility to those parties as well.
If nothing else, it makes the commitment stronger and creates greater incentives to work through issues rather than bail. You can think of G-d and community as additional "accountability buddies" here.
You can say that's stupid, and that it should simply be a commitment between two people and that's it - but you have to acknowledge that the rise of that idea is very correlated with the rise in the diverse rate.
Divorce rate is declining, likely because people aren't being pressured to enter into long-term commitments they aren't ready for by their broader community.
It is interesting to see how conservative ideas are re-packaged to remain fashionable with the time.
I was lucky to be raised by extremely great parents who have never been married and nonetheless have stayed together for 40 years, without God looking over their shoulder.
Seems to me two people with two incomes living in a 1 bedroom apartment is cheaper than one.
Nearly everything is cheaper when you're married: Insurance, taxes, meals, utilities.
What's less expensive as a single?