From Ian Duke:

Thematically, Battle for Zendikar was supposed to focus more on Ulamog and his brood, while Oath of the Gatewatch was more about Kozilek. Ulamog’s mechanical focus was on exiling and processing, while Kozilek’s new thing was colorless mana in costs. So we intentionally dialed back on the exiling mechanical space in order to shine the spotlight on Kozilek.