I think part of it is because all previous south park games have been shovelware pretty much, and all licensed stuff ends up like that too other than the recent Batman games.
But this is motherfucking Obsidian, the best RPG dev in the business. Respect the legends and let's see what they can do. I hope Matt and Trey are working really close with Obsidian and at least end up sharing writing ideas.
They're writing the game. Obsidian isn't.
So you've got an Obsidian game where Obsidian isn't performing the role they're good at.