That's the one thing that really annoyed me about the Christoper Reeve era Superman movies is that outside of Lex Luther and Zod, all of the villains were original creations. To the best of my knowledge, Selena didn't originate from the comics:
https://villains.fandom.com/wiki/Selena_(Supergirl_Film)
https://en.wikipedia.org/wiki/List_of_Superman_enemies
Probably didn't have the rights.
shareAs burk has replied it's probably because of rights issues.
Maybe the producers only paid for the use of certain characters.
Something similar happened with the early Teenage Mutant Ninja Turtles, that's why on part 2 (Secret Of The Ooze) the producers used Tokka and Rahzar, instead of Bebop and Rocksteady.
There are many other movies with those types of issues... there was a Fantastic Four movie which was never released back in the 90s that was only produced on the cheap in order for the production company to be able to retain those rights (I think it was Roger Corman produced).
Even on present day for example Spider-Man even though a Marvel creation was sold to Columbia/Tri-Star (Sony) and now Marvel has to deal with them in order to be able to use Spidey in their Avengers movies, etc.
There are many other example and they are quite interesting to read about.