I think a lot of post WWII success had to do with the fact that both Germany and Japan were high functioning societies prior to the war. While they weren't necessarily democracies they had prior democratic vestiges and high levels of societal and technological advancement.
As y’all would put it, shack. Outside of some selectively pretty nuanced handling with the Japanese by McArthur, we took successes in those cases that were grounded in not starting from square one, and based our approach to nation building from there on in the idea that we can give an opponent democracy and be good to go. It has... not usually worked.