Thing is, Harry Potter featured child protagonists; thus, real-life children were able to identify with the heroes and imagine themselves in their position.
The appeal of He-Man back in the day had nothing to do with identification. But come to think of it, I don't know why it was popular. I mean, I watched Transformers and GI Joe as a kid and understand their appeal as an adult, but I hated He-Man back then and don't understand it's appeal now.
Take Transformers, for instance. The cartoon had certain technical and storytelling flaws, but overall, that shyt was epic. It was essentially the story of an endless war between two competing philosophies: peace and war, creation and destruction. On the other hand, He-Man fought Skeletor because He-Man fought Skeletor. Transformers actually got kids emotionally invested in the characters; when Optimus Prime died in the movie, it was a legitimately traumatic experience for thousands upon thousands of little boys. If a character from He-Man had died, I'm dubious that anyone would have given a shyt--at least not to the same extent.