Additionally, the US arsenal of nuclear weapons after Nagasaki wasn't nearly as continuously supportable as was the island hopping campaign in bringing US firepower to Japan's doorstep.
I am not sure what you mean by continuously supportable. Saipan was the main base of the nuclear planes, as well as the conventional bombers.
The island hopping campaign was necessary for both. The early nuclear bombs were large, and could not be launched from a carrier based plane, or even the B-25 which was carrier based in 1942. (a one time deal, I think all of them were lost on that mission.)