Why not refit cruisers with the Mark 71 eight-inch gun during the war?

Could the tested Mark 71 eight-inch gun be added to American cruisers once the war began?

Not as an immediate measure. Fitting it would remove scarce air-defence cruisers from service for weeks or months of shipyard work precisely when the fleet needed them most.

The proposal creates a fleet-availability problem even if the gun works. The cruisers suitable for conversion were also among the few ships carrying powerful air-defence systems. Rebuilding them around the Mark 71 would require substantial yard time instead of a rapid alongside modification. Combat experience might revive the concept for later construction or refits, but the opening emergency is the worst moment to withdraw the existing cruiser force.