I think there are several different reasons for this (although not all apply to each person)...
1) Moving: Gone are the days when you lived in a 25 mile radius from where you were born. People seem to be moving much more than they used to. And as soon as they move to one place, a few years later they're off to some other destination. I know after high school, I moved out of my hometown to go to college. Then moved again to go to grad school. Then again for a different degree. Then again for my first job. Then again for my second (and current) job. For others it may be the military. Or jobs. Or a cheaper place to live (esp. in this economy). And all of this has been from late teens to mid 30s -- the time when people are getting involved in relationships. It's difficult to have a relationship only to have to move away from that person or have one or the other abandon their own desires for the future.
2) Lack of responsibility: It is easier to live a single life than it is to be in a relationship. Being single, you only have to worry about yourself, whereas when you're with someone else, you have to be concerned about them too. As a single, you can skip town for the weekend without giving much thought, but when you are married, you have to consult with your spouse, children, etc. before going on any type of vacation.
3) Lack of morals: As Beyonce says "If you like it / Then you should put a ring on it" But all too often couples will, well, couple within a few months of knowing each other instead of holding out until marriage (or at least until they are truly committed with each other). People are giving into their desires and passions too quickly before really getting to know the other person. Sometimes they value the other person purely for the sex, and don't really want anything to do with the other person other than as a booty call. And if you can get the sex outside of marriage, then what's the point of being married?
4) Divorced Parents: More and more people are growing up in households with divorced or separated parents. The kids see this and know the hurt they went through growing up being tossed back and forth between parents, or worse left feeling abandoned by a parent that just left for good. Because of this, the kids-grown-up don't want to marry and cause hurt for their own children. For them, marriage it a source of hurt, not of joy.
5) Past Relationships: People may have been married or in a serious relationship in the past, but are now jaded after having that prior relationship fall apart. Cheating, physical abuse, emotional stress, drugs, alcohol... These all can lead to a relationship failing. And after it happens once, what's to say that it won't happen again?
6) Maturity: I also think that some people purposefully wait before they get married. A lot of the reasons why marriages fail is because of a lack of maturity. So because of this, a person may choose to wait until their life is stable and they have matured. He may wait until after his first big promotion when he can afford a mortgage and children. She may wait until after she gets her law degree and is accepted into a practice. Or a person may just know themselves for living too much by their feelings and may want to wait for the wisdom of age to guide them better. Dr. Laura Schlessinger has even been known to urge people to wait until their 30s before marrying; that people who marry in their 30s tend to have a more successful marriage than those who rush into marriage in their early 20s.