I went through the timeline once and it becomes instantly clear when you follow the calendar events. I wrote about it too: https://criticalresist.substack.com/p/south-korea-was-created-from-thin

Japan accepted the prior Potsdam Declaration once the bombs dropped which was not the best deal for the US but prevented the USSR from getting anything too big, since they were not part of Potsdam (it was UK, US and the Republic of China). The US was stuck fighting over atolls while the USSR had started the liberation of Manchuria, and were not needed to win the war unlike what the UK command said in 1942, that they would need Russia's pacific bases to invade Japan. US could have continued fighting as they were approaching their own invasion, but with USSR in the picture, it was better to just accept the deal.