Oh, and a less-cracky headcanon too.
Princess Platinum, like the other two leaders, is shown being really silly and immature in the play, but I figure a lot of that is probably exaggerated, and the historical Platinum was a competent, respected leader. Upon founding Equestria she kept the title “Princess” because she ruled as an equal with the other two tribe leaders, which continued for a time with their successors. Over time the title of Princess became elevated so much that it was respected above King or Queen, which were associated too much with the old world, and so when the current royalty was established they took on that title out of respect for her. Equestria has been ruled by princesses ever since.