Because the Marine version cannot carry the load, fuel capacity, speed nor range for the Navy. Jarheads need about 200 A-10’s and throw in a couple hundred OV-10 Broncho’s.
Those won't operate from the mini-carriers, either. A-10s are generally going to need a paved runway, although it doesn't have to be in pristine condition. And if you have that kind of support facilities available, it probably should be an Army, not a Marine, operation.
Of course, on other threads, I've advocated turning the A-10s over to the Army if the AF doesn't want them. The "B"s are to replace the Harriers (which are basically a 50 year old design).
With in-flight refueling, the range of the “B” and “C” are going to be about the same.