Yeah, there was one great reason: they didn't have atomic bombs in time to use against Germany.

The first nuclear weapons were ready for use by late-July 1945. This was largely a function of having enough nuclear fuel for the weapons.

The Nazis, on the other hand, surrendered in early May 1945. So several months prior to the Allies having atomic bombs.

One could ask: if the US had the atomic bomb earlier, would have they have used them against Germany? The answer is: we don't know. Maybe. There would have been difficulties pulling it off. But it's not impossible. You can read my post about it, linked above.

But as it was, the timing didn't work out.