I'm not American, but my understanding is that this is at least partly about the doctrine of unitary executive power. Trump believes in it, Biden didn't. Trump thinks every part of the executive should be under his personal control. Biden thinks some departments, including the Federal Reserve, whoever overseers elections, and the Department of Justice, should be free from political interference. So the way it worked under Biden is that he appointed the best people he could to the various positions, and then left them alone to do the right thing. He did not put pressure on the DoJ to urgently prosecute Trump or the Epstein files because of this philosophy. Where-as Trump thinks it's fine for him to interfere, campaigned on releasing the files, and now had to follow through. Trump can be held to a different standard because of this difference in philosophy.
This doesn't explain why "doing the right thing" under Biden did not include more timely prosecutions — they could have happened without pressure from the President. I understand some of the timing of the releases was about not wanting to compromise Maxwell's trial or appeals, which have now ended. However, that doesn't explain why more wasn't done to advance other prosecutions. On this I tend to go with the conspiracy theories. The DoJ didn't want to rock the boat to the extent that would be necessary.