The issue is probably that BBC-DVD have a certain budget for 'Added Value Material' for each release. If there's any sort of reconstruction of apart four on Tenth Planet, that's the AVM budget gone. Whereas if The Aztecs only has a few more extras for the re-release, then Airlock can be the rest of the AVM for that release.
Plu, of course, 10th Planet was going to sell anyway, whereas lots of us were probably going to skip a double-dip on the Aztecs. So in brutal commercial terms (and BBC DVD is a business, not a goodwill operation for fans) this was inevitable. Sure, I'd hoped the people at BBC DVD were still fan enough not to do it, but ultimately... it's fans in their time off, businessmen when earning their salary - and looking for ways to justify their continued salary.