Way more work. To make something like those Fallout games or Morrowind/Oblivion sellable to modern consumers, you'd basically need to rebuild them from scratch with all new assets. Skyrim looked good enough that upressing it, adding in some newer engine features, and cranking up to PC max settings was good enough for a remaster, but the same wouldn't fly for the other games - and besides, if you want something on that level, Xbox BC (4K 60fps for all but New Vegas, and even that is 60fps) and the PC versions are available.
Perhaps at some point doing a full on remake of morrowind with some more modern mechanics added in would be interesting, but would be a massive undertaking - a mod group has been trying to do it in Skyrim's engine basically since that game released, and are still only half done.
