Time for my contrarian speculations!
I can think of several good reasons why there were no extra Sailors created for the series.
They weren't allowed to create any. This possibility might seem odd given that the old anime diverged very much from the manga in many respects, to the point where it is an alternate continuity in of itself. However let's look at all the original characters that were created. The vast majority of them are victims of the day, monsters of the day, recurring supporting characters, or villains who were swiftly dealt with at the end of their arcs/movies and never mentioned again. There has never been an original character for the anime that was either a clone or a distorted version of the Sailors. That may have been because they weren't allowed to create additional Sailors. This might have been spelled out in whatever agreement used to agree to adapt the manga; perhaps only Takeuchi was allowed to create new Sailors.
They wanted to keep in line with continuity. Imagine you were a fan in the 1990s, before the Outers were introduced. Let's suppose you had your own vision of a Sailor Saturn. She was a ring-throwing, ring themed giant girl with orange hair and red eyes, who liked to dance, party, and play the guitar. You wrote fanfiction for this character, you drew this character. Now imagine if Naoko Takeuchi either made a rule completely invalidating your character, or worse yet produced a version that was completely different from what you had envisioned.
As a fan you would have been devastated, having created all this fan art and fanfiction for a character who was noncanonical. But as a fan you would have only wasted your own time and your own efforts.
Imagine then you were part of the production company and your team was tasked with creating a new Sailor character. Let's suppose this new character was then scripted and animated into even a special. Now imagine Naoko Takeuchi, not only ignoring this character but then once the license gets back make sure the character is never seen again.
Congratulations! You not only just wasted the effort and time but you also wasted a company's money, money that cannot be recouped.
They feared an overcrowded plate. Out of all the new characters added for the anime, note with the vast majority of them came in; the first season. There isn't much room for any more characters than what we got. Would it be possible to create a new Sailor? Of course. Would it be possible to devote enough time to her and the canonical characters? No.
The possibility was precluded by the change in showrunners. As many die-hard fans know, Junichi Sato was the director for Sailor Moon and the first part of R while Kunihiko Ikuhara took over the remainder of R as well as the entirety of S and SuperS. When you look at the vast majority of the original recurring characters, they were introduced in Sato's run. Maybe Sato would have added more Sailors but Ikuhara refused to go that route.
They didn't think of it. By far, this is the most likely reason but I'd like to add that we're looking at this series retrospectively. It's easy for us to envision a "Sailor Anything" because we have seen the series from beginning to end. However, it is not clear when watching the series from the beginning that there is a Sailor "Celesial Object" for every object, Looking at the series, one would easily assume that the Sailors were connected to the Silver Millennium as a group; it wasn't clear even at first that there were even any more Sailors than the main five. At most, there would be enough for the Solar System...but that's it.
This is evidenced by the very next group of enemies in the anime after the Dark Kingdom being aliens. If they thought it was possible for there to be extra-solar system Sailors, you could be certain En or an En-like figure would have definitely been one. There may have also been a Sailor Nemesis/Sailor Black Moon if they thought it would generate toy sales...but they probably didn't think there was.
Sorry, but I wish Naru was made a Sailor Senshi, but not a good one. Since Toei was taking liberties with the story, they should have made Nephrite use the Dark Crystal to morph Naru into some type of evil Senshi (like Dark Mercury from PGSM) because I really wanted to see how they would pull off the conflict with her and Sailor Moon...and if she found out her identity..it'd be great. I'd love to see how Nephrite would pull the strings in that situation.
That's one of my weird fan fantasies.
My fantasy is even weirder. I would have had him used the Dark Crystal to sense that Naru was actually a latent Youma (technically a Youma-hybrid...yes, that would mean Morga was
actually her mother) and that is the secret power he sensed from her. However, I'd still keep his budding morality so he'd actually have some reservations about awakening her power...only for her to transform anyway out of stress and a desire to protect "Masato Sanjoin" when Sailor Moon confronts him. Of course, like PGSM, I'd give Sailor Moon the ability to heal transformed people from the start so that would be resolved at the end of the episode...for now..and later in the arc she would find out about Sailor Moon's identity...