At the time Germany surrendered, we were afraid of what it would take to end the war with Japan. It was fully expected that we would need to invade and occupy most of Japan before they would surrender. The thinking was that we needed Russia to invade Japan from the North; in fact the Russians did invade a small northern island and largely kept it after the war.

Additionally, the Germans had shown the difficulty of trying to invade Russia -- and Germany did it before Russia prepared for the war. After a few years of war, including us supplying them, they would have been much tougher to invade. Combine this with a war weary American (and British) public, it is very questionable if they would have supported a continuation of war that was likely to produce lots of casualties, particularly against an "ally".