The short answer is "because the Cold War happened".

The somewhat longer answer: After World War II, Germany was divided into occupation zones by the victorious Allies: the United States, Great Britain, France, and the Soviet Union. Relations between the West and the Soviet Union deteriorated as the Cold War developed. The American, British, and French zones were united into capitalist, democratic West Germany while the Soviet zone became communist East Germany. West Germany soon became an American ally against the Eastern Bloc and East Germany a Soviet ally against the Western Bloc. Finally, in the 1980s, the Iron Curtain collapsed and Germany reunified under the government of West Germany, which is now the present German government. So basically, any remaining hostility between the U.S. and Germany was superseded by the Cold War and the emergence of the Soviet Union as a common enemy.