The U.S. essentially managed Japan after WWII and was involved in restructuring the government. Possibly more importantly, Japan was seen as a buffer in Asia against the spread of communism, and the U.S. had incentive to both prop up Japan economically for trade, but strengthen Japan as an ally in the Cold War. 

And strengthen Japan they did.

All the hullabaloo you hear about Japan being a post-WW2 economic miracle through the power of 70 hour workweeks & Japanese super creativity was actually thanks to the endless foreign investment of money and free technology sharing.
The key turnaround for Japan's fortunes was the Korean War. Being the closest base to South Korea, the West threw endless money into Japanese infrastructure to keep producing ammo & food and delivering them to South Korea. That's how modern South Korean cuisine incorporates cheese and spam.