I agree with @compmom. Why should we assume that the only people who can ultimately be successful in life are those who are ready to go to college at age 18 and who are able to finish a degree in 4 years? That’s an awfully arbitrary requirement. Some students just won’t be ready till a few years later, or they need to work for a few years first, or they need to take longer to graduate.
I think asking “why do some kids crash and burn while others succeed [at college]” is like asking “why are some 13-year-olds already 6 feet tall while others are still the same size they were at age 9?”