there actually were some talks about dividing japan during the war, but when truman got to power, he pretty much ended those talks (it's well known that while fdr was much more amiable towards stalin, truman distrusted stalin). the postdam conf highlighted how far apart the western allies and the ussr were regarding postwar spheres. to avoid the mess in central/eastern europe that was developing, truman gave the entirety of the japan islands under the command of mcarthur and scap. it should be said tho that the british empire contributed some forces so it wasn't purely an american occupying force.

as for why didn't the other allies challenge it, well the usa was clearly the main power that defeated japan, the roc wasn't in a position to occupy or demand anything as they were still in a brutal civil war with the ccp (a war they would soon lose), the british empire already had their hands full with re-establishing control over singapore and their other colonies (in addition to dealing with what was unfolding in europe), and the ussr simply wasn't in a position to challenge truman (militarily speaking, they had nothing in the pacific that could challenge the western allies). stalin wasn't happy, but as their sole contribution was a short invasion of manchuria (which was very effective), it wasn't like they had much of a moral imperative to occupy japan. stalin's attention was also obviously much more focused on what was going on in europe. so truman got his way so to speak. but the animosity between the west and the ussr was already in development by vj day and not giving any part of japan to the ussr was one step towards what would become the cold war.