Probably because we haven't gotten an upgraded game since BW2.I mean the last 3 games have been either the first-entry to a new gen(which have always been light on content) or a remake of an older game(which had more end game content than the original although not as much as the upgraded version of the original).
Generation debuts are always light on content, but XY and SM have been light on content even by those standards (especially XY). And there was no real excuse for ORAS to have less content than Emerald. So that's not all there is to it. The issue is Game Freak's focus on making the games accessible, they've constantly based their decisions on making the game for a wider, less dedicated audience (or in other words, they want to cash in on the mobile market), and that philosophy has affected multiple aspects of the game including difficulty, replay value, gameplay depth, and region design. And fans are just getting sick of it.
1. What qualifies as a big upgrade?
1. Major changes to the region and story, more specifically, a large amount of new areas and content (about the same as BW2), multiple changes to old areas, and more differences in the story than just a slightly different ending.
2. A large expansion to the regional dex (at least 100 new Pokemon in the dex)
3. Multiple new features that add at least 5-10 hours of extra content to the game.
2. How do we know that GF didn't make big upgrades?
We don't know for sure, but there's multiple red flags that indicate that it's probably not going to be that substantial:
1. The short development time. This game has only been in development for a year more than SM. It's difficult to see them making major changes that quickly.
2. Game Freak's philosophy towards making the game mobile-esque. They probably have no interest in making the kind of large scale changes people expect them to, and they're probably not going to have a lot of depth in the extra content.
3. Adding a BW2 size expansion would require new islands, which are unlikely to happen because the four island system is important to the game's lore and would require a new Oricario form and a new Tapu, the latter of which is especially unlikely.
4. This game is being described as an "alternate universe", which could easily describe a third version.
3. So not having Z is bad, but having USUM is bad?
Well I don't think people actually want third versions, it's really hard to justify paying for the same game twice. The likely reason people want third versions is because they see them as mulligans, an opportunity for Game Freak to correct the mistakes they made in the original game. So you really have to look at this on a case by case basis. XY had a lot of problems with lack of content and a storyline with more loose ends than a coil of sisal rope, and a lot of people wanted to see an update to XY that addresses those issues. Now SM has some of the former, but not quite as much. Additionally, USUM is in a transition period with people expecting a Switch game soon and with the Stars rumor going around people were all too happy to buy into it and ended up let down when it was proven false.