As a developer, it's probably money. But most people here jump to the negative conclusion about money because they have no idea about game development. When I say money, I mean a lack of funds to justify the upscaled production of the game, which can no longer justify the costs. When you first start as a dev, it's 99/100 cases a hobby. That hobby grows, and you start earning some money. Let's say the game makes 50% of the money you put into production back. If your game costs 100 dollars a month to make an update, you get 50 back. That's 50 dollars for a hobby, no big deal. If you're on a normal income, that's doable.
Now, let's say your game is upscaled and costs 1000 dollars a month to make, and you earn 500, so now it costs you 500 dollars a month to keep producing. Many developers never see their game making any profits, only increasing losses, even if their subscriber counts increase. The increasing costs might still be at the same ratio as the income, but in absolute terms, they can no longer afford to sustain development.
Of those developers who do make some money (me being one of them), most (and again including me) work for de facto less than minimum wage. The number of hours sunk into each update isn't worth the money most of us earn from game development. So, it remains a personal project that, in the best of cases, pays for itself and maybe accumulates a little money that can be invested in better hardware.
Only a tiny sliver of bad actor devs make it big and then start to slow development to leech off their subs. Most of us never reach this stage, and of those that do, only a minority are these bad-faith developers. They do attract a lot of attention, though, but unfortunately, it rubs off on normal developers as well.