Because Lariansaid that their aim was to get all the player's handbook stuff in first, so that's the only stuff we can say with anything close to reliability is going to be in the final game. Having said that though, I do think Piff is being a bit too uncharitable since it's not like Larian isn't ALLOWED to use stuff outside PHB, or that they don't want to. PHB content is simply their first priority, so if they really want some sort of evil-themed paladin, then I fully expect them to include one that already exists before they make another.
