I can think of a reason. Quad confused the buying public. A lot of people remembered the double stocking of mono and stereo, and they weren’t completely compatible, so plenty of people felt that the quad and stereo versions would not be compatible. But to make double inventory was also expensive, so to reduce cost and confusion, they only produced one mix, and relied on word of mouth to get the niche sales.
Unlike you, I don’t work in the recording industry, so that’s speculation on my part, but I’ve heard that story from people who would know.