The few.
The proud.
The only ones who were available at the time.
When the royal budget is blown, Luna needs to show that the disasters that have so drained Equestria's resources aren't going to continue. Reforming the Lunar Guard on a tight purse means that she doesn't necessarily get the ponies that she would have chosen, but how bad could it possibly be?
They're reckless, dangerous, they weren't even supposed to be here today, and they might well be Equestria's last hope.
(Featured and flattered!)