Maybe they just forgot to mention it to McArthur. After all, they didn't mention it to president Truman until April 24, 1945, and even he did not know if it would work until July 16, 1945, which was just 3 weeks before they dropped it..
https://www.history.com/this-day-in-history/truman-is-briefed-on-manhattan-project
Maybe a Vice President didn’t need to know
Those leading our military forces needed to know what weapons were available and what was coming
It had major impact on military strategy
You have to keep in mind the mind set of some of our military commanders. Curtis Lemay, for example, was the model for "Jack D Ripper", in Dr. Strangelove, the SAC Commander who went crazy and started a nuclear war. After WW2 was over, the president directed that all nuclear weapons be put directly under his personal control. LeMay tried to hold back a few, "just in case". It didn't work. LeMay went on to run as George Wallace's running mate. MacArthur advocated nuclear war against Korea, and presumable China as well, during the Korean war.
Besides. If word had gotten out to the Japanese that we were putting the invasion on hold, they would have assumed a much stronger negotiating position. In fact, it was recommended to Truman that he blow up an unpopulated Japanese Island instead of a city. Truman rejected the advice, correctly, in my opinion, that the Japanese would have assumed that we were not prepared to actually use it against the people of Japan. He was not afraid to use it, which is one of many reasons that Truman is my personal hero.