You're absolutely right in that it is hard to make sense of surround releases in general, but I think I can shed some light on some of the specific examples you mentioned.
Concerning Elton John's unreleased surround mixes, they were finished around the time of the commercial downfall of the SACD format for popular music, and even though there's been a perfectly suitable alternative for high-res surround that's come along since then (Blu-Ray) Elton himself has not shown enough interest to care about getting them released. That could certainly change at some point though.
When it comes to "Roxy Music", the latest I heard through the grapevine was that the delay is due in part to Bryan Ferry's dissatisfaction with some of the artwork and designs, so they may be looking to redo some of it before they finally release the new edition.
Finally, we know now that the holdup on "Tales" appears to have more to do with the recording licensor than it does with the record licensee (Panegyric) and the artist, both of whom still seem willing to want to release it at some point.
None of these issues plague Steve Hackett's catalogue. You have a willing artist and a champion for his music inside of the major label (Mark Powell) who has been able to revitalize and reissue his recordings for the fans to enjoy, and I still feel confident that the same could definitely happen this year for both "Tales from Topographic Oceans" and "Roxy Music". (though the same cannot be said for Elton's unreleased 5.1 mixes. I've already thrown in the towel when it comes to those.)
