Because it’s prohibitively expensive to be a third-country carrier with a skeletal operation between markets where you are not a “natural” consideration. Being a U.S. (or any third country) carrier for example between Tokyo and Bangkok is a difficult proposition as you usually will only have one flight a day in the market at odd times (to support your long-haul connection) while the local carriers are able to corner the higher-yielding local traffic with more frequencies, and scale on at least one end.

PA and NW being dominant in TPAC travel in Japan was a post-WWII relic that became increasingly difficult to maintain once the local carriers started growing.