Well the arc makes numerous references to how your character is an incarnate, and the plot doesn't really work unless your character is incarnate to boot (channeling incarnate power to some end, taking on foes that are beyond non-incarnates, etc.). There is no story way for a Praetorian who hasn't gone through the rift enclosure to become an incarnate. SCoRE removed some of the restrictions on incarnate leveling that were present on Live, making it so you don't have to be doing any specific content (all of which requires transitioning to Primal Earth), but that was purely for player convenience, and no story reason for lifting that restriction was ever written.