Presumably by using the same rationale as was applied by Conservatives when demanding the FBI investigate Clinton for mishandling restricted government documents, coincidentally the exact same offense as Trump is apparently suspected of, and which they duly did.
Which difference would you like to get into, the one where the FBI never actually investigated Clinton, despite literal bleach and hammers being used on hard drives, or the one where Trump in his capacity as former President can argue that his handling of said documents falls under his purview?
If asking for a little bit of consistency out of people is coocoo, as opposed to naked tribalistic DERRR DRUMPF impulses in which the full power of the State is unleashed on a political enemy under the flimsiest of pretexts (or if having a natural distrust towards this apparatus, as if this isn't the same FBI that didn't manufacture FISA warrants that they knew were based on lies against Trump campaign) then yes, I guess I'm freaking crazy.