I think you are mistaken about your premise — the US did participate in massive bombing campaigns against Germany during World War II, though it is true that the bombing campaign against Japan evolved to be ultimately more extensive and indiscriminate (a trend that began against Germany but then became intensified against Japan). There were several reason for this, including a difference in tactical situations (the US was already committed to a land invasion of Germany, but not yet Japan), an evolution of capabilities (the B-29 proved to be better at area bombing than precision bombing), and cultural assumptions (there is a sense that the US felt that Japanese civilians were less worthy of their consideration, for racist reasons, and they also made arguments that the Japanese war industry was more dispersed among the civilian sector).

If you are asking about the atomic bomb, the US did not have an atomic bomb ready to use while Germany was still an enemy in World War II. Germany surrendered in May 1945. The atomic bombs were not ready until late July 1945.