I don't have numbers to back this up, but I think there was a significant number of habitual Republicans motivated to turn out just to vote against trump, yet voted R downballot out of their habit. They didn't think of consequences and certainly wouldn't ascribe blame to R for the lack of government effectiveness. Couple this with a larger turnout of cultist Republicans, many turning out to try to keep the first black woman out of the White House, and many more turning out to defeat 'socialism' (as the fearmongers of Fox define it), and you have a red wave trying to counteract the blue one. Of course, you also have to take into consideration the gerrymandering in some areas, the selective reduction in polling places, the voter purges, voter intimidation, USPS tampering, and likely electronic tampering where possible. It can take a blue tsunami to counter a red wave under those conditions.
Short version: There was bipartisan enthusiasm for voting trump out. There was lesser, partisan enthusiasm for downballot candidates.