A second theory I have:

Besides the "High Elf" theory listed out above, there may another reason: The "Ultimate" and "Collectors" editions of the Mordor Expansion have a 30-day VIP code with them. When first released, people with a lifetime account or an on-going VIP sub were selling those like hotcakes, for as much as 7500 gold. And people were paying it, too.

With everything up to Minas Morgul now included in VIP, that 30 day code becomes INCREDIBLY lucrative to get a pretty vast amount of gold. If Mordor Ultimate would ever go on sale, one could buy it 4 times on 4 freebie accounts, and sell the VIP code 4 times for a buckload of in-game gold.

So there might just be a reason related to that as well to not sell it any longer, because that certainly had been an issue in the past with people selling VIP codes in world chat for up to a year after Mordor was released. And they are going to want you to buy VIP with actual real cash, not buy it in-game with in-game gold.

So with the High Elf also in mind, they would have to maybe re-pack them into versions without the High Elf, and without the VIP codes. It might also just be an issue in general, as they changed store systems since the release of Mordor, and those VIP codes may no longer work.....