"It would permit damaging disclosures that would reveal sources and methods of surveillance potentially nationwide," according to the government's lawyers.
The Justice Department's argument for why broader releases would be impractical pretty much stops there. The rest is either redacted or addresses First Amendment legal arguments.
Ultimately, the government's practicality case boils down to the idea that if the terrorists know which services the government isn't watching, they'll be able to elude eavesdropping much more easily.
But companies that issue transparency reports do so only after the surveillance has already taken place. And the fact that practically every tech company has received a non-zero number of data requests means that the risk of surveillance is omnipresent no matter what service the terrorists choose. Just because Google fielded 21,000 data requests in its most recent reporting period compared to Microsoft's 37,000 doesn't make Google a safer choice for would-be criminals.