The DOJ policy is to afford exec branch the benefit of the doubt on classified docs. Many of the Biden docs were acquired when he was Senator. Senators often take stacks of classified docs home (except TS/SCI, of course). The DOJ is more concerned with their retrieval, and whether or they will cooperate in their return, so they do not prosecute for possession, alone, unless:
1. There is willful, deliberate attempt to take the documents without intending on their return and that can be proven. PLUS the fact that....
2. They obstruct the government's efforts to retrieve them.
That is NOT the case with Biden and Pence, but is, indeed, the case with Trump.