it's a combination of economics and cultural. marriage is serious to some groups and patriarchy places value on women not being "used up". if you are a recent immigrant, of course your culture will be intact and will be enforced. over time, however, your american decendants will drift more toward the american norm for their class level.
economics can mean that abstinence wont really improve your life so you might as well do what you want when you want to. in america, upward mobility is a myth, over time, more people have figured that out, so for instance, white american out of wedlock births continue to rise do to economic factors. marriage among americans is beginning to be associated with wealth, so our culture has become to pretty much wile out if you know you arent really going to have much money in life.