Game is great, it just had everything against it to sell, not to mention MS is probably happier with the outcome as they claim raising Game Pass subs is more important to them, so game sales are irrelevant for this title and I think "number of players" are completely valid here or else they wouldn't be putting it on Game Pass.
1. It came out after two big bangers, both being only $50
2. It came right before Switch 2 launch
3. Game is $80
So yeah, people already spent their money and are saving it for the new console, but also...
3. Eternal harmed the brand a lot. Eternal fans don't like it being too different and those who didn't like Eternal don't want another disappointment
4. Mandatory RT makes some people believe the game performs like those other games that just use RT as a gimmick (thus you have many people claiming it should be "optional" thinking it's another gimmicky implementation, no it isn't), many don't even try to
From the reboot this is my favorite one, closely followed by 2016 and trying to make my mind and pretend Eternal didn't exist, so game quality is NOT the reason it is not selling, Game Pass is, most people preferred to play it there for the reasons above.