Main Idea: Truman wanted to end WW2 quickly after Germany and Italy surrendered. (Japan did not)
Details :
They did not want to have a full scale invasion of Japan as fatalities would be high. Nor have a full scale blockade of Japan as that would take too long.
The U.S.S.R was also trying to make a move on Japan, and the U.S did not want the U.S.S.R to have influence over a beaten Japan.
The U.S actually had no idea what the Atomic Bombs would do to cities, as they only tested it in a desert. So Truman decided it was no different from bombings of Dresden/Tokyo