It isn't a lore reason at all. They just wanted to give us another thing to grind post-msq and we ultimately have six "Scions" (counting the unofficial Ryne) with us, so they made the trust window account for six npcs and called it a day.

Whenever we're redoing content in the DF we're essentially going back in time / revisiting it with the Echo, so it's not like there should be any valid "lore" reason for why we can't take the Exarch to the Switch anymore.