Selective Prosecution
I know many think that the prosecution of Trump is political but just consider how valuable these documents might be, we can't just have anyone reading them, people need to have a security clearance. Granted Trump as president did have that security clearance, it's not like he has a crackhead son who is reading them and then trying to feed his habit by selling secrets to foreign countries. However, you have to keep these documents secure. Do you realize if they are not guarded carefully someone could hack into your computer or sneak into your garage to steal them. Now granted he did keep them locked up and guarded by the Secret Service, but still this is not a trivial concern. No, the issue is when the government comes asking for them they must be made available, granted his lawyers did do that and answered every request and when the swat team came he wasn't at home so the lawyers responded to the warrant. But the real charge is not that he had these documents, obviously he has the right to them and charging him with that would bring up some very thorny questions about Biden, and Clinton and Pence, etc. No, the issue is that when the government asked for them was he quick enough to hand them over. Granted they got them the same day as they came for them unlike Biden's documents, but that is simply because Biden didn't know where they were all being kept.
So I hope this explanation clears up any confusion about why it was so important to prosecute Trump but not Biden or Clinton.