I don't ask this question to get into a political discussion, just enquiring as to the legal situation.
The second volume of the Mueller Report lists ten or so possible charges of Obstruction of Justice. For each, he lists 1) what the law would require to be proven, 2) how his evidence fits that law, 3) the likely ways the indicted would attempt to fight the charges and 4) why it wouldn't be likely to work.
As we probably are all aware, a memo within the Department of Justice's Office of Legal Counsel opines that a sitting president cannot be criminally charged while in office.
However Obstruction of Justice I believe has a five year statute of limitations. So, it would seem to allow prosecution in 2021 or 2022. Why would not the next Attorney General (to be clear, from the Biden administration) bring these charges after Trump left office, but before the statute of limitations came into effect? I don't recall any mention of Mueller's Vol. 2 charges all being too weak to bring.