I can give you a simple answer - in this generation, most of the functions and benefits in marriage are replaced with substitutes. Practically speaking, marriage is a social institution that pools resources, makes a living and bears children, it wasn't just a union of two individuals, but two families, two clans, two tribes, two nations, you know. Nobody was so foolish and naive to think of marriage as a license to have intercourse and intimacy, as we were taught by the purity culture, it was primarily about politics and economy, not love. Any romantic love was just added bonus. And for women in particular, marriage was the only way to reach a higher socio-economic status, hence the term "hypergamy", i.e. marrying up. A successful marriage was the telos, the ultimate goal of life for most people. Also, marriage was a rite of passage, a transition from adolescence into adulthood, this is universal in all cultures, including indigenous tribes.
These days, however, there're a myriad of options and opportunities to meet these material, physiological and psychological needs, most people are able to sustain themselves, men don't need women to cook and clean, women don't need men to provide and protect, and with reproductive technology, you don't even need a spouse to have babies! For those unfortunate and downtrodden who can't have their needs met, they'd seek help from communities, organizations, government instead of their own families, many of them come from a dysfunctional family in the first place. There're also a myriad of other great pursuits in life and other ways for indulgence - including sexual pleasure. In essence, marriage as a social institution is functionally replaced by these other social institutions.
So as you can see, marriage is no longer substantiated by these basic material demands, and since it's by definition a lifetime commitment, it has become rather a burden too heavy to bear, that's disciples' conclusion of Jesus's teaching of marriage - "If such is the case of the man with his wife, it is better not to marry." (Matt. 19:10). The only reason to get married is a spiritual pursuit based on the biblical understanding that marriage is the holy matrimony, a sacred union between a man and a woman, an earthly reflection of our ultimate heavenly union with Christ. In the creation week, all things created by God was good, the one and only thing bad was man's solitutde - "it's not good for man to be alone!"
in the early 80s played a role too.
