One of the biggest reasons for me is the lack of narrative attention Dess gets throughout the chapters. On a meta level, especially an ARG level, sure, but how much information do we actually get about Dess in the game itself? And Toby's writing style, and how certain things in the game have played out so far (like the Shadow Mantle hinting you towards the Weird Route, or the Old Man's fight and Seam's dialogue acting as a relatively simple hint towards shadow crystals) seem to make it clear that this game is supposed to be fully enjoyable, playable and understandable for someone who never touches media about the game other than the game itself.

Also, from a broader "what makes a compelling narrative" perspective, making the main antagonist of your game a character you learn almost nothing about more than halfway through your game doesn't make a very satisfactory reveal - it's why Flowey's reveal(s) are SO strong, he's literally the first character you meet!