I think the marketing suits at Filmation & Mattel realized that while He-Man & the Masters of the Universe was geared for boys, it was also attracting young female viewers as well, thanks to Teela, the Captain of the Palace Guard, who had been posited as a possible girlfriend for He-Man. Not only that, but Queen Marlena, Prnce Adam's mother, had been presented in a positive light, too, as a role model.
So it was that when production began on season 3 of He-Man, Mattel & Filmation expanded the series' self-contained universe, and introduced She-Ra, Princess of Power as a companion series. Naturally, He-Man guest-starred in the opening story arc, which explains how Princess Adora, Adam's twin sister, had been working for Etheria's evil ruler, Hordak. The siblings eventually meet, and Adora sees the light--literally.
Unfortunately, the bloom was falling off the rose for He-Man by this point, and She-Ra lasted just 2 seasons, replaced by Bravestarr in 1987. While He-Man has subsequently been revived on 2 different occasions, She-Ra was not brought back, although there was talk when Cartoon Network cancelled the last He-Man series that She-Ra would finally return, nearly 20 years after her debut. While He-Man has also appeared in comic books, among other things, She-Ra hasn't seen the inside of the printed page at all.
Scope out the episode, "The Price of Power".