The same can be said of Teela and He-Man.
He-Man: no origin in the cartoon, we're told he's important, the hero, and most powerful man in the universe. It's like Mattel said "Kids, you like He-Man" and the kids went "okay!"
Teela: Trainer of Adam (even though they're the same age), Captain of the Royal Guard (even though she's like 16-18 and they're all grown men who have been in the RG for years...very believable there

)
So...how did either of those characters earn the right to be leads in the franchise? Teela (and Bow) are the staple "damsel in distress" characters. Filmation just gave them a little more credibility and strength. It's just like Steve Trevor in the 70's Wonder Woman.
But yeah, I think She-Ra earned the right more than Teela. Adora was kidnapped and when she learned the truth, she changed EVERYTHING about her life. She had a "hold on now, this isn't right" moment and is fighting an entire WORLD of villains, and not just tag-teaming alongside this bigger, stronger hero or a whole army. Most of the time it's just her vs. them.
