The ponyverse owes more to Princess Leia Organa (the late Carrie Fisher) than you might have thought:
Princess Leia is the direct ancestor of Princess Twilight Sparkle, by way of Disney’s 1990’s renaissance. She was the first wildly popular “princess” who spent the whole story doing decidedly non-princessy things. Sure, she was still getting rescued by some guys, and she didn’t fly a star fighter herself, but she was tough enough to withstand torture, she was smart and tactical, and she wasn’t taking any bullshit.
It’s not fair to say that this was a drastic change from the idea of a princess. At least as far back as Sara Crewe in the 19th century, “being a princess” has implied being made of tougher stuff than a lot of people give them credit for. And for adults, Eowyn beat Leia to the scene by a couple of decades. But as far as active, competent princesses that could be successfully marketed to little girls, in the 1980s Leia was the proof of concept.
From there, Disney (the world’s largest producer of princesses and princess related accessories) picked up the torch and gave us Belle and Mulan and Tiana and made approximately ALL the money off of them. Which is why Hasbro always wants more pony princesses.
And, not entirely unrelated, we have M. A. Larson’s Pennyroyal Academy, where princesses are too busy saving the world to spend any time as distressed damsels.
Girls in the MLP:FiM demographic seem naturally attracted to the idea of being princess-y; we have Leia (and Twi, and others) to thank for the idea that being princess-y doesn’t at all preclude being a badass.