Because the bombs were not ready. (I assume the question is specifically about the use of a nuclear weapon; the US had been bombing Japan with conventional weapons.) Remember, the devices represented...
To get rid of our Naval fleet.
4 years
so the US citizens would support an attack rather than join the war unprovoked, because of attempts to remain isolationist
Korea was forced to split upon Japan's surrender per the Potsdam conference agreement: http://en.wikipedia.org/wiki/Korean_War