It's pretty simple - Biden wasn't president when it stored his records, Trump was. All classification authority emanates from the President, not senators or vice presidents.
To do so, Trump would have had to have done said declassification. Something he notably did not do. In fact, he also didn’t turn them over when explicitly asked when he left office.
He also lied repeatedly about keeping them, and kept them (and handled them) well after he was out of office and no longer president. He also explicitly directed employees of his which were not cleared for the material to move and handle those records.
He also kept them in a place easily accessible to folks who were paying him money (personally) at Mar-a-lago and went to some trouble it seems to destroy evidence related to who may have seen them (security tapes, etc) when the FBI came looking.
And that is ignoring the issues like a huge (and unprecedented) spike in CIA assets being killed/disappeared related to this material once he was in office, the Kushner loans from the Saudis that seems related to this material and other information, Kashoggi’s murder (right around the time of the Kushner loans from the Saudi’s BTw), and the whole fiasco with Flynn (appointed national security advisor by Trump) with Russian ties. And Trump’s repeated statements trying to shake down the Ukraine gov’t and explicit statements that he would direct US foreign policy against what are clear US interests and towards widely stated Russian interests. Like withdrawing from NATO, or removing US support for Ukraine.
In contrast, Biden appears to have kept some records at home in a location that was not widely accessible, and he returned them when asked, and as part of normal processes common to transitions. Not ideal.
But I’ve heard no accusations (not even flimsy ones), let alone seen any statements or actions from him, that even imply he is working against US national interests, or the nation has suffered any actual harm related to those actions.
So not even on the same planet as what Trump has admitted to doing, let alone what seems to be quite apparent that he has been doing flagrantly.
If Biden committed a meaningful crime? Impeach him, remove him from office, and lock him up.
Trump has been convicted of numerous felonies, not even counting Jan 6th and numerous other acts committed in full view of the public, and is still rattling around free as a bird and undermining US public interests.
Why is he not in jail? Why is he still out on bail, especially after making clear statements (before and after trial) that he has plans to flee the country?
Oh, and Biden, being president later, could have even gone back and declassified those records he had in his garage to cover his ass - but didn’t.