Nightmare Moon's return was not without incident and injury. Those that were chosen by the Elements suffered and were pained in the attempt to save Celestia's younger sister. While they succeeded, the trial left scars.
Princess Celestia sees this as everything going as planned and is ready to use the new Bearers for anything she needs taken care of.
Princess Luna thinks the group should be protected and only used when the situation is at it's most dire.
A compromise is made and Luna is allowed to form a team to deal with threats that do not require an orbital strike from a friendship canon. Provided she can find those willing.
With the memory of Nightmare Moon still fresh in every ponies mind, Luna is unable to find ponies who are willing to serve her readily.
So what is a Princess of the Night to do?
Not recruit ponies obviously.
[ Story has my usually Lewd jokes and comments but no sex and thus the warning is here. Story not affiliated with TGaP or SiS. It's just yet another idea i had. ]