Even though there are few successful marriage stories (mostly of older people or on internet), statistically, it is not common.
I would summarize today's reasons into few categories:
1. The institution of today's western marriage - nothing good for men, tons of duties, both legal and social, frequent divorces, horror stories, extremely expensive...
2. Women (generally) lost their femininity, so they are not attractive to men and middle class men are not attractive to middle class women because of status
- many younger people stopped serious dating altogether, not just marrying
3. Younger generations are "broken" (both economically and psychologically) - nobody knows what is marriage for, what are the roles of each gender, why to have children, how to behave in marriage, how to solve parenting problems or relationship problems, people do not have money to have children or even their own house, everybody has some mental problem or health problem or other complications, most people have divorced parents or unmarried parents, so no good examples and patterns and no multi-generational support for a new young family...
4. No reason to actually date or marry, in an economically developed society. In the past, marriage was a way how to survive, how to get some money or status etc. Children were needed to help you when you were old. But society works differently now and marriage or children are frequently rather obstacles to achieving a better economical situation.