Well we don't know what the legal agreement was for these different companies. It's just as possible that WotC bought back the copyright or that DriveThru RPG has cut a deal with MWP and or White Wolf.
The latter seems extremely unlikely, because then the publisher would be listed on DMs Guild as "MWP" or "White Wolf", instead of "Wizards of the Coast". Also, we know that the Ravenloft licence permitted White Wolf to sell existing stock of printed 3rd Edition Ravenloft products only for a limited time (six months?) after the licence to produce new products expired. Thus, to be able to start selling those books again now, in any format, would have required WotC to grant them a new licence, which seems improbable.
I couldn't find any direct evidence that the licensing deals for those properties meant that the copyrights for the materials reverted entirely to WotC, but we do know that was the case for all of the D&D comics, which is why WotC was able to subsequently license IDW to reprint older comics initially published by other companies. This indicates to me that WotC's standard licence for any material based on their campaign settings ensures that all rights for that material reverts to them after a period of time.
And as DriveThru RPG and Paizo are both competing for the same PDF market (and as DriveThru penalises companies who sell on both DriveThru and Paizo in order to give them an incentive to avoid Paizo) I think that Paizo would not be inclined to allow DriveThru access to any magazines it still has the rights to.
For both the Paizo-era and earlier Dragon/Dungeon magazines, my theory is that the lack of releases on DMs Guild is unrelated to any licence with Paizo. I think the copyright for the D&D material in those issues has long since reverted to WotC. However, there is additional (non-D&D) content in most issues for which the copyrights never belonged to either WotC or Paizo, for example, the comics. For this reason, you'll find that the electronic versions available for sale at Paizo are not complete issues. They do not contain the adverts, and some issues are missing certain comics. As I'm sure you're aware, this situation is even worse for the TSR-era content, where the electronic publication rights for the articles is, to say the least, rather complicated.
Thus, there is a significant "hassle factor" involved in selling those issues via the DMs Guild. It's not a matter of just scanning the issues, but also ensuring that only the content for which WotC actually has the copyright gets included. My guess is that WotC has looked at the risk/reward and decided simply not to bother with content for which the rights are not 100% clear.