I wonder why there are many Germans in America who don't speak German.

Because the German descendants of the United States went to the United States earlier, many of them moved to the United States before World War I, and they were all poor people in Germany, and they didn't quite agree with their local culture. In contrast, the French have a stronger sense of cultural identity, while the southwestern United States is a Spanish-speaking country, and there are many Spanish-speaking new immigrants.