You misunderstand the issue, at least re: Steely Dan. Although it started as an independent reissue label, Rhino is now a subsidiary of Warner Music Group (WMG). Rhino only releases reissues of WMG catalogue material. If it's not in the WMG catalogue—which includes the labels Warner Bros., Elektra, and Atlantic, among others, as well as some releases that originated on other labels (such as the Chicago and War catalogues)—it's not fair game for Rhino. This is why Rhino would never release Steely Dan material of any kind, quadraphonic or not. As to why the Steely Dan quad mixes won't be rereleased at all, that'll be down to Donald Fagen, who is the final decision maker for the Dan catalogue.
As for actual WMG catalogue titles, although the minimum bar is met for Quadio, there may be additional hurdles for certain artists and albums. Generally, living artists have some say in what gets released. So, if—for example—Don Henley doesn't want the Eagles' quad mixes rereleased, that's that. In other cases, estates of deceased artists may want to prevent a particular release. The only way to guarantee an album is releasable, assuming the master tapes are available, is if all rights now reside with WMG.