Trekkie was originally a derisive term coined by some fans of literary science fiction. For many decades science fiction conventions consisted primarily of literary science fiction fans discussing their favorite books and hearing stories/panels from their favorite authors.
Star Trek was a media form of Science Fiction, and a lot of its fans began to attend Science Fiction conventions to discuss various aspects of Star Trek including its production, and organizers began to court and invite various persons who had been involved with Star Trek to attend these conventions and they became more than just literary science fiction conventions.
Many fans of literary science fiction didn't care for these developments, and used the term to differentiate themselves from a group they didn't consider "True fans of Science Fiction".
That's why the term Trekker came into use by Star Trek fan as themselves, as they didn't care for the derisive term some literary science fiction fans had come up with for them.
Star Trek was a media form of Science Fiction, and a lot of its fans began to attend Science Fiction conventions to discuss various aspects of Star Trek including its production, and organizers began to court and invite various persons who had been involved with Star Trek to attend these conventions and they became more than just literary science fiction conventions.
Many fans of literary science fiction didn't care for these developments, and used the term to differentiate themselves from a group they didn't consider "True fans of Science Fiction".
That's why the term Trekker came into use by Star Trek fan as themselves, as they didn't care for the derisive term some literary science fiction fans had come up with for them.