Why did it take until 2018 to sink most Clinton seat Republicans?

[ElectionsGuy]

This is something I'm generally still curious about. In 2016, most Republican incumbents in suburban districts that trended Democratic hung on, and way outperformed Trump. This is seen in Senate races too (Ron Johnson in WOW, Toomey in SEPA, Kirk in Collars) too. In 2018, they mostly did worse or performed on par with what Trump did.

My premature explanation would be that in 2016, it was easier to separate traditional Republicans from Trump, as you could vote Republican down ballot but just not for president. Also, many congressmen and senators distanced themselves from Trump and didn't say whether they'd vote for him in 2016. In 2018, Trump wasn't on the ballot but almost all congressional races were nationalized and tied to what people thought of Trump. Republicans at this point were forced to back Trump. Another explanation could be the Republican party is now 'Trumpified' with Trump being president in a way it wasn't yet in 2016, in that almost all Republicans now perform like Trump at the congressional level. But this is essentially just a result of what I said in the sentence before.

Any other thoughts? While this could justify, I'm curious if there were any other factors at play.