Actually the transfer of the Arabian Oryx's status from "Endangered" to "Vulnerable" is a fairly technical one under the criteria operated by IUCN for categorising species on its "Red List" and shouldn't be regarded as indicating a significant increase in numbers -"stable" or "increasing slowly" are the sorts of terms used . Also this technical change in status was foreseen as long ago as 2008 - just after the Omani site was de-listed and even referred to numbers as in 2006.
See this link for more details on how the categorisation (and its change) is determined and how many creatures remain "in the wild"/in various states of "captivity". Its revised status is apparently justified solely on the 1000 at 1 site in Saudi Arabia. Furthermore, in many locations there are doubts as to whether they should be regarded as being "semi managed" rather than truly "wild".
http://www.iucnredlist.org/apps/redlist/details/15569/0/fullMore relevant to the Omani World Heritage issue is that the "stabilisation" or "small increases" in numbers are not being achieved in Oman itself. Although Oman was in the forefront of activity in the Middle East during the 1980s for reintroducing the species into the wild it has since been overtaken by more successful activity in other countries. I can't see Oman being the slightest bit interested in getting "World Heritage" status back for the Reserve. It made its position pretty clear in 2007. As I understand events, it didn't really "ask" for delisting - it just refused to go back on its decision to reduce the size of the reserve by 90% (thus facilitating oil exploration) and accepted the resultant "delisting" with equanimity (unlike Dresden which fought it tooth and nail)! How important the lost 90% really was to the OUV of the site I don't know but it seems clear that Oman was no longer an example of successful activity regarding Oryx reintroduction and that numbers had significantly reduced - due to "poaching" it was said. But I can't really believe that Oman couldn't have solved a poaching problem within the reserve if it had really wanted to!! I guess Oman decided that continued inscription of the Oryx Reserve wasn't that "strategic" to its future (It certainly wasn't a major element in its tourism strategy unlike the cultural inscriptions) whilst UNESCO decided that it just couldn't ignore what was happening and that, with the growing success of the "World Heritage brand" it was time to make an example (though it had previously, and since, turned a blind eye to infractions in many other sites!!!).
All rather unfortunate for my "WHS count" as I "lost" one of my "visited sites" - albeit that it was more of a "technical" visit than a worthwhile viewing of the Oryx -
http://www.worldheritagesite.org/sites/arabianoryxsanctuary.html