I'm just assuming it's because his dreadnought is his throne world after he and all his subordinates turned it inside out and that the only reason we didn't kill him permanently the first time during the regicide mission even though we were on his ship is because we weren't ascendant, so we couldn't access the correct plane of existence to kill him. Also on a side note, that blast he does to destroy the awoken fleet was simply the act of him expanding his throne world.