Marriage is a sound economic decision actually. A dual-income household without kids would have a waaaaaaay easier time paying rent (or, alternately, the mortgage) and affording the occasional luxury. Research even shows that married men tend to earn more (a lot more, over the long term). I would presume that all modern people know how to use birth control to, say, put off the first child for at least the first 3-5 years of marriage, or for however long it takes to get their finances in check.
The wedding issue is also a rather arbitrary one. For whatever reason people nowadays often want crazy expensive weddings but a simple ceremony at one's local church, where the bride perhaps wears a hand-me-down dress, and then a family get-together at someone's house where everyone brings one food item and a beverage, with a very modest honeymoon (if at all), would be manageably affordable.
The only real barrier I can see here is, say, underemployed people who still live at home, but save in cases of mentally ill people I can't see that being an issue all the way up to age 40.
The actual reasons are:
1. Unrealistic expectations that one can or must find a "sole breadwinner for a family of four" job before they tie the knot
2. Inflexible and wasteful thinking about finances and budget that lowers the perceived limit of how much they can "make work"
3. Lack of motivation to actually go out there and establish/continually invest in a meaningful relationship with someone of the opposite sex
4. Perceived lack of need for marriage because "I like living alone and I can just bust a nut when I get bored"
5. Desire not to be limited sexually, or to have to give up hobbies and recreational activities (especially after kids are born)
6. Insecurities due to personal shortcomings like obesity or an STD
7. Desire not to have to work longer hours (especially if, say, you're working 35 hours a week in a Wal-Mart and live in a trailer park)
8. Longer times spent in college, and therefore in putting off a full-time career
9. Fear of the marriage ending in a divorce (especially for men)