Very soon after the program started, due to the emergence of the Cold War, the western powers and the United States in particular began to lose interest in the program, somewhat mirroring the Reverse Course in American-occupied Japan. Denazification was carried out in an increasingly lenient and lukewarm way until being officially abolished in 1951. The American government soon came to view the program as ineffective and counterproductive. Additionally, the program was highly unpopular in West Germany, where many Nazis maintained positions of power. Denazification was opposed by the new West German government of Konrad Adenauer, who declared that ending the process was necessary for West German rearmament.
Fascism is a social disease that is injected by the bourgeoisie into the petit-bourgeoisie during times when capitalism is in crisis. They do this through propaganda and economic warfare like austerity coupled with tax cuts for the rich. This results in an economic downturn that hits the “middle class” the hardest (relatively - poor people are also fucked, but they had less to lose in the first place). Economic woes coupled with divisive and authoritarian propaganda and — boom — fascism.
So how do you stop it? Well, ultimately by dismantling capitalism and removing the power of the bourgeoisie. Otherwise fascist crises will just keep being manufactured.