Why 10-inch County cruisers erase the 12-inch cruiser niche

What purpose would a 12-inch large cruiser serve if County-class cruisers displaced 15,000 tons and carried 254 mm guns?

Very little; the 10-inch cruiser would already cover so much of the role that a 12-inch design would lack enough differentiation.

A 15,000-ton County-class cruiser armed with several 254 mm guns would already occupy much of the space between a treaty heavy cruiser and a 12-inch large cruiser. Moving to the larger weapon would provide too little extra differentiation for the additional ship size and cost. The overlap becomes still greater if the 10-inch cruiser carries a numerous battery. Under those assumptions, the 12-inch design loses a clear tactical and procurement role.