@Akera93 Probably a combination of having mediocre writing (not a dig at the series, just saying maybe the in universe story was told a bit less well) and also just being insanely long.
Worth remembering that the protagonist of what he read redid stuff a ton, right? Like when he dies it resets or something? So imagine if it was something like redoing these 18 chapters 70 times, I can see how that'd shake off a lot of the readers, if say, you were a million words down and only finally getting to chapter 18 equivalent.
Still though it's true that ultimately it comes down to 'plot device' I'm sure there'd be at least a few other people probably, but honestly even 10 might be a bit optimistic depending on exactly how tedious it got.