It seems you're answering all your own questions.
- In the "ought six" family, the .280 is stuck between the more established .270 and 30-06
- Among 7mm/.280 options, the 7mm mag offers superior ballistics, while the 7mm-08 has less recoil and can be chambered in a short action. Also, some people still like the 7x57 Mauser round.
The .280 is a very good round. It just happens to have a lot of tough, well-established competition with more ammo options. Regarding the question of why 7mm-08... I'd say because it differs more from the 30-06, .270, and 7mm mag than the .280 does.