In a tiny announcement on Ubisoft's blog, it was revealed that South Park: The Fractured But Whole has been delayed until early 2017. The blog post reads as follows:
South Park: The Fractured But Whole will now launch on Xbox One, Playstation 4 and PC, calendar Q1 2017. The development team wants to make sure the game experience meets the high expectations of fans and the additional time will help them achieve this goal.
So no official reason, really. It certainly isn't to perfect the game's title, because boy oh boy have they knocked it out of the park in that department.