“Why is the Japanese government such more keen to Delta rather than AA? Does JAL really fit in DL/NW-Skyteam alliance? “
Maybe it’s because the Japanese government has a soft spot for Northwest (now part of Delta). Northwest helped set up JAL in October 1951. The first JAL passenger flight was operated by a crew leased from Northwest.
But if JAL were to join Skyteam then I can see conflict with fellow member Korean which operates a large hub at nearby Incheon airport at Seoul. Incheon rivals JAL’s hub at Tokyo Narita as a gateway to the region.