The rights to Isaac Hayes Stax albums reverted back to his family a few years back. This has been great from a fan perspective because it's led to the issue of previously-unreleased music from his golden years (primarily on the 4CD
The Spirit of Memphis box set in 2017) but the double-edged sword is that the more people that have a say in the control of music, the more challenging it is to license it, if they even want to at all.
More recently, the estate entered into a
strategic publishing partnership with Primary Wave so maybe they'll be more eager to monetize assets like the quad mixes.
Concord hold the other Stax quads, along with the Fantasy/Prestige/Milestone material, the Vanguard catalog, and a few other bits and pieces, but they don't seem to do much licensing outside of to Analogue Productions, which issues jazz from some of those catalogs at a snail's pace. I'm as eager as anyone to see these quad mixes reissued and I've reminded Mike Dutton about them over the years so I think it's a case of "if it can ever be done, it will be done."