Because we didn't want it to fall under Soviet domination. That explains The Marshall Plan, as well.
Communism was a threat in Japan, through their trade unions and the communist party. But the companies co-opted the trade unions and Japan is always suspicious of outsiders.
Also, they wanted to copy a winner, so they copied the US after WW II. Deming wasn’t listened to in the US, but he was highly esteemed in Japan. Toyota Production System? Deming’s ideas adapted by the Japanese.
I hear you. The point is what other country would do what we did? Yet we’re still reviled around the world.