Both parties make promises to their bases. But while the Democratic establishment more or less tries to make good on those promises, the Republican establishment has essentially been playing bait-and-switch for decades. And voters finally rebelled against the con.
One possible explanation is the decadence of the G.O.P. establishment, which has become ingrown and lost touch. Apparatchiks who have spent their whole careers inside the bubble of right-wing think tanks and partisan media may suffer from the delusion that their ideology is actually popular with real people. And this has left them hapless in the face of a Trumpian challenge.
Worth a read.