I think that might be because She-Ra is a more inherently interesting property, at least to the outside observer. That’s not to denigrate He-Man or Masters at all. I credit He-Man for awakening the Muscle Mary within and for giving me a love of storytelling and fantasy, but I always connected more to She-Ra than He-Man, bulges aside.
He-Man is a fairly standard adventure scenario: He-Man is the good guy, Skeletor is the bad guy, and He-Man spends his day trying to stop Skeletor from taking over Eternia. He-Man is a supercop with super deltoids who works on behalf of the ruling authorities. She-Ra, on the other hand, is a freedom fighter trying to TAKE BACK the world from the evil that’s enslaved it. It gives the franchise a slight shadowy undercurrent, even if the political implications of She-Ra’s actions are sanitized and candy-colored. In that respect, I think the animation did way more with the property than He-Man did: based on the descriptions of the She-Ra action figures, I never would’ve extrapolated the plot of the cartoon.
Let’s not forget the fact that She-Ra is a woman. While she’s not the first female character to headline her own mainstream action series, she’s probably the first that a lot of people think about. Milla Jovovich even cited her as a major influence in a recent interview. There are plenty of iconic TV action heroines that came before, from Emma Peel to Elektra Woman and Dyna-Girl, but She-Ra had her own franchise with her own name on it. That’s a pretty interesting achievement, and aside from a lot of female-led action series that came before (Honey West, anyone?) people still reference her and revere her as an icon.