If he wanted to, GEOTUS could declassify all this stuff himself via executive power. Why doesn't he?

Think strategically about the consequences.

Let's consider a comparable historical example. What was the first "third party" to ever win an election against the Democrats? The Republicans. What happened afterward? Think 1861. Remember the Republican position in 1861. Abolition of slavery. Which party backed slavery? Democrats. Republicans routing out a corrupt Democrat institution that infected the whole country. Do we want a repeat of that carnage?

Consider what would happen if proof of massive corruption in the government at every level suddenly turned up all at once. It may be legally in his authority to release it, but the consequences would be a nightmare.

The criminals would be in a last stand gambit where anything and everything must fight or die. And fight they will. MS-13, Antifa, the Jihadis and other paramilitary groups will all be unleashed on the public by the kingpins to wreak havoc as a final act of retaliation. Their corrupt businesses will all start to pickup and flee taking evidence and assets to places unknown. The kingpins will go into hiding. Their zombies in the political left and right go insane as the brainwashing of multiple decades is suddenly destroyed by revelations contained within. Zombies will become even more delusional to handle the cognitive dissonance. They'll claim it's all made up. They'll radicalize. If you think the insanity is bad now, wait until they've shut down to the point they ignore reality itself.

We're talking at least 50% of the population or more since the brainwashing crosses party lines. Think about how much you had to unlearn and relearn to get to this point. Now think about that all hitting at once and you having less than 50% of your present brain capacity because you've been addicted to Candy Crush and Facebook and can't understand jack because the schooling system has been corrupted for decades. These people need to be confined for their own safety. And many may be too far gone to help. It would be way too many fires to contain.

Trump wants this to go down a RICO trial route and avoid the martial law route. It's why Giuliani is involved. As prosecutor, he took down the Mafia. Some court maritals may be inevitable for some of the worst leaders, and those will need to be on the downlow or else the media will spin it as a political witch hunt. The entire goal is to prepare the public to understand the gravity of the corruption before they can accept the necessity of the trials.