It's also probably to avoid confusion with the EU DLC.
Look at the way Europa Universalis DLC are named. They're generally EU : <name>. If the game was EU : Rome 2, that'd potentially confuse people.
Having it as a new IP solves that issue, as DLC for the game can now be things like Imperator : Veni Vedi Vici for a war and conquest DLC, or Imperator : Alea Iacta Est for a civil war DLC.
Look at the way Europa Universalis DLC are named. They're generally EU : <name>. If the game was EU : Rome 2, that'd potentially confuse people.
Having it as a new IP solves that issue, as DLC for the game can now be things like Imperator : Veni Vedi Vici for a war and conquest DLC, or Imperator : Alea Iacta Est for a civil war DLC.