The challenge for Multichannel is the added production costs to make such releases without a matching increase in sales when that feature is added.
There are hundreds, if not thousands, of Quads all done and dusted (and gathering dust!) all mixed in Surround and ready to be utilised, with no further costs required for remixing etc.
The main overhead (beyond licensing) would seemingly be tape transfer of the 4-ch masters and remastering the Quad.
You then stick the Quad on a Hybrid SACD, together with the lucrative Stereo in Redbook and Hi-Res/DSD.. Ta-da!
This is not anything special, taking a Single Layer and "upgrading" it to a pricier Hybrid, its a Hybrid by definition, so can't be any added expenditure to the label there - can there?
Is the cost for the transfer and mastering process of the Quad tapes so onerous it makes including the Quads (as with the BS&T Hits, Weather Report & Return To Forever AF Stereo-only SACDs) a non-sequitur?