Unfortunately, I think the grimdark stuff is just a phase for a lot of people.
When you’re a teenager, and more moody, you have a lot more to get off your chest, and your interests happen to line up with My Little Pony, which often makes something great. But, years later, when you finally work through those emotions, you don’t really have a need to get it out anymore, and oftentimes these people know that their work will suffer if their heart isn’t in it.
This is why they move on from grimdark, too. They don’t like feeling miserable. When there’s more lighthearted things (like the actual show) to be enjoyed, suddenly death, surrealism, horror, and whatever else feel a lot less appealing. At least, it’s something they don’t want to be exposed to all the time.
I can’t account for that being the reason why every grimdark artist stopped drawing it, but that’s what I think the most common reason might be.