This is going to be hard to answer I think. Personally, I think that this is multi-faceted:
1. COVID led to record high voter turnout that was going to be hard to replicate for either party.
2. For low propensity Democrats, they were probably unenthusiastic about their candidate. Joe Biden was extremely unpopular with the Democratic base when he dropped out, and they replaced him with somebody who consolidated that support within the party but still not all of it. Having a President that is clearly in decline due to his age, followed by a secondary candidate that was not selected through a primary - and not getting much needed name recognition - depressed internal Democratic turnout. Republicans did not have this issue. For the record, I think any candidate they picked would've suffered the same as Harris did. As annoying as it is to have a year long nominating process, it certainly helps get your name in the news cycle that reaches all Americans eventually, no matter how much they don't pay attention.
3. Inflation brought in new low propensity voters for the opposition party. This happened everywhere in the world; Trump probably ended up with several million votes from the people who did not participate in 2020. Thus, inflation acted much like COVID in driving voter turnout to one party single handedly while having the opposite effect on the other party.
For 2024 so far, Trump managed to win all 7 swing states but will end up only winning a single Senate seat in them. Similarly, despite causing a MASSIVE swing in New Jersey, for example, Republicans aren't really going to do damage there down ballot. I actually don't think they'll flip a single Republican held seat there despite the largest gains in that state in a generation. Similar events unfolded in other states. In Nevada, Trump will win but Democrats will retain all their house seats. This was the case in pretty much all the states.
Thus, millions of voters in those states showed up and voted for Trump but didn't vote down the ballot.