Those were mostly purple years; the United States isn't—at least, when functioning approximately according the the Constitutional rules, which has suddenly become an important qualifier—a unitary executive dictatorship.

Because presidents didn't used to act like kings. It's extremely common for the House and/or Senate to swing opposite the President:

https://history.house.gov/Institution/Presidents-Coinciding/...

There's also the problem of Democrats being absolutely terrible at organizing and uniting.