This goes back all the way to World War II. PanAm already had strong connection with the U.S. government. Northwest was awarded Asia route by the U.S. government after the war as a reward for contribution during the war.
After WWII Japan was occupied by the U.S. for 7 years. Japan was not allowed to have commercial airlines for 10 years. Because of the U.S. occupation PanAm and Northwest could fly in and out of Japan freely to any destination, hence 5th freedom right to rest of Asia destinations.
Because of geographical location of Japan, Tokyo worked out great for PanAm and Northwest as stopping point for flights from the U.S. to rest of Far East countries. Back then there was no aircraft which could do non-stop U.S.-China.
Then fast economy recovery of post war Japan has made Tokyo as profitable destination for PanAm and Northwest. Example, back in ‘60s and ‘70s PanAm and Northwest could not make profit by Seoul, South Korea and the U.S. traffic alone. But flying Seoul-Tokyo-U.S. made better sense business wise for PanAm and Northwest.
After World War II, there were Korean War and Vietnam War which Japan was important logistic point for the U.S. military during those wars. PanAm and Northwest having a base at Tokyo made military charter operation during those wars logistically important for both airlines.
Also, bilateral commercial aviation agreement established during the U.S. occupation after the war heavily favored the U.S. airlines. During 1980s 80% of the U.S.-Japan traffic was carried by the U.S. carriers where Japanese carriers only carried 20% of the U.S.-Japan traffic. Another reason Tokyo has worked well for PanAm and Northwest.
All above reasons, Tokyo has become key destination for PanAm and Northwest. Having hub at Tokyo (first at HND then NRT), bringing passengers from the U.S. to Tokyo and have passengers continue to perspective Asian destination. 5th freedom right of PanAm and Northwest meant PanAm and Northwest could carry local Japan- rest of Asia traffic without any restriction. These made Tokyo a very good hub for PanAm and Northwest.
Where United inherited Tokyo hub from PanAm and Delta inherited Tokyo hub from Northwest. Even today with high corporate demand of Tokyo means high paid business class and paid first class demand at Tokyo. Especially Delta not having any alliance partner at Tokyo, Delta has tried Seoul, South Korea where Delta has Korean Air as a partner. After merger of Delta and Northwest, looks as Delta’s plan is to stay at Tokyo. Delta has opened new Asia operation center at NRT and also opened new aircraft maintenance hanger at NRT in past few years.
It is correct about DL and UA competing with Asian carriers, especially with rise of LCC airlines for intra Asia traffic. Looks as DL and UA both are focusing more on the U.S.-Japan traffic at NRT rather than NRT-Asia traffic these days.