Due to significant political and social differences. For example, why has the value of a high school diploma decreased more in the US than in Europe? Why does the US have more of a problem with for-profit colleges? Why does the US have such a rapidly rising cost of college (regardless if the cost is paid by students or by the state)?

If I had to give a single root cause, it would be how we vote (the US system leads to a far stronger polarized two party system than Europe has).