Popular refers to anything non-classical, which includes pop, rock, jazz, R&B, soul, etc.
Vocalion and Epoch have made their reputation (and a good one at that, look at the happy customers on their facebook page) over the last 20 years primarily releasing classical, soundtrack, and easy listening music, including the artists you appear to be turning your nose up at in your post. They'd be foolish to neglect their core demographic going forward as that's what got them to where they are today.
By the same token, they'd be foolish to ignore easy money making opportunities (ie reissuing rock quads on SACD) if they were available, just because they had some stylistic preference for non-rock music. This isn't the case at all. As I said some months ago on this thread, Michael Dutton told me himself that he's not averse to releasing rock quads - hell, he cut his teeth working at Morgan studios in London as a tea boy when Jethro Tull was cutting Songs From The Wood there.
The thing is, when you get in to reissuing rock and pop quad titles, you're overlapping with one of the most desirable periods for titles (1969-1977) that reissue companies go after. So that means that when Vocalion is trying to get anything rock/pop/jazz/soul/R&B from the 70's they're going up against all the other UK CD reissue labels, which include some heavy hittlers like BGO, BBR, Cherry Red, Expansion, Ace as well as loads of other smaller ones. When any of these labels license a title for reissue it can take it out of circulation for other potential licensees for 3-5 years, or even more - some of the BGO titles that are licensed from Sony they've had in print for more than 10 years.
Between the email requests D-V get, posts on their facebook page, requests here (they do check in from time to time) and suggestions from myself they have a very good idea of what people are after. They're not living in some alternate universe where they're scoffing at the quality of Aerosmith and the Winter brothers whilst simultaneously plotting to release the complete quadraphonic oeuvre of Ray Conniff. They're filling a gap in the market that other labels aren't covering, and also expanding their horizons to try and offer a broader selection of titles. I think when they released their first issue of 5 or 6 quad SACDs at the end of 2015, they didn't entirely realise that there was a network of people like us who were looking for all sorts of different music in quad - they just added the SACD quad layer as an extra to boost sales in their core customer base, and because Mike Dutton is a fan of surround sound himself. Now that they know how much was produced during the quad era, and what people are after, they're doing their best to connect you the customer with what's sitting in the major label vaults.
If you want to know what's available to them, a little research goes a long way. Take a look at Aerosmth for example - look up their three albums that were released in quad on discogs for example and you'll see that they've never been reissued by anyone other than Columbia/Sony. In fact, the stereo versions of the albums that are on CD currently appear to be the same ones that were released in the early 90's, which would suggest to me that there's a financial or contractual issue preventing re-release - surely someone like MoFi would have had a go by now if it was possible. As for the Winter brothers, you can look them up on amazon UK, you'll see that the majority of their albums have been reissued in the last few years by either BGO or Music On CD. This is what D-V faces when it gets in to the arena of reissuing rock quads, as I'm sure all CD reissue labels do. It just burns us more, obviously, because when they reissue something on CD in stereo we lose the ability to get it in quad. That isn't to say there's nothing available - as licenses expire, new things become re-available on a yearly basis, but you have to work with what those things are, not to mention compete with other labels to get them.
On top of that, even for titles that they do manage to secure licensing for, locating quad master tapes that haven't been touched since Watergate isn't as simple as phoning up Iron Mountain and saying 'send it over!'. Vault searches can take (and have taken) months or even longer, and spanned continents and still not turned up things. For all the quad things that D-V have released so far, there are just as many that are either on hold or not happening because they couldn't find the tapes.
I haven't even touched on the time and money involved in all of this, but suffice it to say that paying an upfront guarantee to a major label, legal and contractual costs, along with the costs of stereo and quad tape transfers, stereo and quad SACD mastering, stereo CD mastering, hybrid SACD authoring, disc replication, artwork, printing and packaging aren't cheap. I think if D-V were in a position to choose at will what they'd release, they'd give us everything we wanted - they're not avoiding it or holding it back out of a perverse desire to make Danny Davis an international sensation 40 years after the fact. I think when you consider all the hoops a label like D-V (or AF when they were doing quad) have to jump through to get even one quad SACD out it starts to seem like more of a minor miracle than anything. They might not be releasing exactly what you want, or on the timescale you want, but they are trying hard to unearth stuff from the major label vaults that would presumably never see the light of day otherwise.