Because he probably faced far more serious and multiple charges. Why would he make a plea?? Because he is guilty otherwise he would have fought it.

He's already a war criminal.

pleading so,the punishment is probably the cheapest resolution

Play nice and the prosecutors may recomment a fine and maybe no sentence.

Million dollar book-deals can cover a lot of friendly fines

and although I did appreciate his decision to sorta stand up to trump, he remained a craven opportunist in doing so -- and was therefore a candidate for the trump tactics that turn all the other craven opportunists into cowardly trump brown-nosers.

He's almost certainly technically guilty of some classified document mishandling; it's probably something no trump loyalist would ever have been prosecuted for; and it's trivial compared to trump's mar-a-lago bathroom documents capers

And once he got on Trump's shit list, I'm sure that mysteriously there were things that was overlooked resulting in his facing charges.

As for pleading guilty, lots of innocence people plead guilty for lots of reasons. Maybe one of them is just to quit fighting the litany of charges that the Prosecutors can continue to bring against you.

or so in prison, and likely die in prison. By pleading guilty, he'll pay a six or seven figure fine.