In Vietnam not only because of the lack of RWR was the issue.
Check the loadouts of F-105, A-6, A-4, A-7, F-4s and other planes what carried and from where.
The range + loadout for the F-104 was simply not in the scale what US demanded. They built large and expensive plane to themselves compared to rest of the world in the '60s.
The smaller F-104, A-4 and other planes were just heritage of the past. Until the F-16 I cannot recall any US supersonic fighter which was under 10 tons empty weight.