I suspect in the more recent cases, someone screwed up and forgot the composer was supposed to be credited on the covers under their composer agreements...
Elmer Bernstein was credited as I recall on RAMBLING ROSE
Lee Holdridge wasn't credited on the SIXTEEN DAYS OF GLORY
In both cases both the studio and labels fouled up.
The question always reminds me of Page Cook and 55 DAYS AT PEKING, which had the credit in tiny white letters on a black background -- thus allowing him to obliterate the offensive name with a heavy black marker!
I've recently been revisiting the FSM release of Miklos Rozsa' wonderful score for Time After Time and was shocked to discover Rozsa's name totally absent from the cover?