Trump's personal behavior (bragging about sexually assaulting women, condoning violence from his supporters, refusing to condemn from KKK, etc.) alienated him more so from well-educated voters than from less educated voters.
Lots of good reasons mentioned in this thread I think. The correlation, state-by-state (weighing each state equally and counting DC as a state) between the percentage of residents 25 and older with a bachelor's degree or higher, and the percentage of voters who voted Trump, is a jaw-dropping -0.85.
Not surprising in general, but amazing how strong the correlation is. What were the biggest outliers? (I'd guess Nevada would be more Democratic than expected)