It’s a mixture of a lot of factors I imagine but republicans clearly badly underperformed.I get that there's a case to be made that this is a pretty bad result for the Republicans, as historically parties with the Presidency usually suffer mid-term, plus there's a massive inflation problem, so one would expect a competent opposition to make significant advances.
But what I don't know is whether, in those past historical cases, the starting point for mid-term elections was such a deadlock situation in both houses. Though the Republicans seem to have only made tiny gains - which reflects badly on them, and on Trump, at this point - the objective situation remains a legislature split down the middle. Could be a lot worse, but it's still not great, it seems to me. I don't know if in those past cases of opposition parties making gains, what the starting point was, i.e. maybe in some of those they were way behind to start with?
As far as the rest all I can say is the US system is poorly designed and has always depended on ideologically diffuse parties to function. Now that we don’t have those anymore we end up with gridlock most of the time.

