A long time ago in a galaxy far far away, there was a little planet called Neopia on which the Sailor Neopets were created. They were the grandmothers of today's Sailor Neopets, and they were the originals; they got their powers directly from the Faerie Queen, rather than inheriting them from their parents.
Back then, of course, Tyrannia hadn't been discovered, so there was no Chomby; Meridell hadn't been found, so there was no Draik; the Poogle and the Meerca had not been introduced; why, even Mystery Island hadn't been stumbled across, so there weren't any Kougras! Since these pets didn't exist (as far as most of Neopia knew), the Faerie Queen of that time didn't give anybody the power to become the sailors for those pets.
Fast-forward to the present. The original Sailor Lupe has a granddaughter, Leslie, who is Sailor Lupe today. The original Sailor Peophin has a granddaughter, Zoe, who is Sailor Peophin today. The original Sailor Meerca - but wait, there WAS no original Sailor Meerca! She can't have had a granddaughter (or grandson) if she didn't exist. Therefore, there is no Sailor Meerca, along with a handful of other sailors: the "pending" soldiers.
If there's ever going to be a Sailor Meerca, she'll have to get her powers directly from the current Faerie Queen, Fyora. However, Fyora's a pretty young Faerie Queen, and she doesn't have the time or know-how to go out and find the perfect people to become the sailors. So guess who gets to do the job?
If you said "the current Sailor Neopets", you're right. Our beloved characters will have to pick people they know and trust and feel would be good at the job. And they won't be traveling the globe to search for appropriate people; they'll pick out of their friends and acquaintances. Which is why if you want a pending senshi, you have to introduce her into the story as a regular person and have her meet at least some of the sailors. They can get introduced in the first or second series.