I think the sales or marketing (or whatever you want to call it) strategy at CBS for quad changed in 1975.
I was listening to one of those
WNYC Men of Hi-Fi programs from early 1976 that had an interview with Stanley Kavan who worked for CBS, and one of the first things he said (and I'm paraphrasing here) was that "1975 was a tough year for quad". I think once the excitement of the new format wore off, people became more selective about what they were buying, and as a result CBS became a bit more selective about what they were issuing.
From 1972 to 1974 I think pretty much everything they issued on SQ LP they also issued on Q8, but in 1975 you start to see some SQ LP-only quad releases from them, and they're almost all R&B titles - this MFSB one, The O'Jays'
Family Reunion and
Survival, Phoebe Snow's
Second Childhood, the Isley's
Harvest for the World, Johnnie Taylor's
Eargasm, etc. Record labels don't do things for no reason, so my guess is they probably sold considerably fewer Q8 copies than LP copies of soul, funk and R&B albums in the years before 1975, so when the belt-tightening came in '75 it seems like they were the first casualty.
I bet it was also significantly more expensive to dump unsold Q8's than LPs - with LPs they could melt down and reuse the vinyl, but you can't do the same with magnetic tape. I've also noticed (and many other people have pointed this out too) that the CBS SQ vinyl pressings in the later years of quad (especially '76/'77) are really poor, like maybe they were skimping on quality there just to get them out - I don't think a good sounding copy of Billy Joel's
Turnstiles (1976) or the Isley's
Go for Your Guns (1977) exists for example, they all either have horrible inner groove distortion, are pressed off-center, or both.