Historical and geo-political (not just NRT, servicing Tokyo Haneda before NRT was built and the pacific region in general).
In the case of Delta, it's because Delta acquired NWA. NWA had a long history in the Asia Pacific Region, especially Japan.
https://en.wikipedia.org/wiki/Northw...rk_development
United got a piece when it purchased PanAm's pacific routes in the 1980s.