The basic of artillery don't require a specific weapon, so the old ones work fine for basic training in the art. For the same reason, the Russians still used WWII era ZIS-3 76mm guns and D-44 85mm guns up to the present.
The 76mm in the Sherman was our standard medium tank gun in Korea, with only the 90mm guns in the M26/46 to keep it company. The Marines did have a number of Shermans with 105mm howitzers, though. The 75mm guns in the M24s were considered too weak to do any good.