Geoff Peterson wrote:
None of those people played aliens, so it doesn't really answer the question.
Some WKFs at the time did indeed
play aliens. John Watts, who is mentioned a few times in Bjo's "On the Good Ship Enterprise" was an Andorian (coincidentally, the same Andorian Christopher Bennett named for my Andorian alter ego in "Ex Machina). Paula Crist, who was a popular cosplayer at conventions before the term was even invented (and later played "Baby" in "Baby: Secret of the Lost Legend") appeared as Worene, her own alien creation. Don Fanning was a Zaranite. JoAnn Christy was a Vulcan. 70s convention entrepreneur Steven Lance was a Rhaandarite.
As to why nobody else ever wore those costumes... maybe they were constructed to fit a particular person?
Are you talking about the rubber masks/hands or Starfleet uniforms? It's not like the heads were attached to particular uniforms.
The fan extras were selected to fit the ready-made costumes at their casting call. One uniform type exception: the tiny white female jumpsuits specially made for Susan Sackett, Bjo Trimble and Louise Stange. But again, they played humans.