There's a lot going on developmentally speaking around that age. People don't develop at the same rate and going from living with parents to living at college is a BIG CHANGE.
So there are as many different reasons as there are runners, but one factor I think we see a lot is the transition from being micro-managed to asking the runners to manage their own lives.
For most runners, this is going from having their parents prepare their meals, make their choices, protect them from dumb kid stuff. It's GREAT to have the freedom to make one's own choices, but face it: the choices that lead to running success are a LOT less fun in the short term than the choices most college first year students make.
Look at Coach Joe Newton from Elmhust, IL. He's trained TONS of successful HS runners. Mostly by telling everyone what to do and enforcing the rules. When these kids go places where coaches treat them like adults, they tend to not do as well. This isn't a criticism of the kids or the coach, just pointing out that what works for children doesn't work for adults.
Also, like others have pointed out, top level NCAA training involves soaking in a lot of high mileage aerobic training that my take a year or two to 'soak in' as it were. Coaches (well, GOOD coaches) don't train 1st year students for first year success. They train them for 4 year success which means sacrificing short term success for long-term payouts.