I think at the time, fantasy fiction was really popular. Stuff like King Arthur, Merlin, etc... 1963 was the release date for Disney's Sword in the Stone 4-5 ye

I think at the time, fantasy fiction was really popular. Stuff like King Arthur, Merlin, etc... 1963 was the release date for Disney's Sword in the Stone 4-5 years before Mr. Roger's Neighborhood in 1968. I imagine kings, queens, knights, and dragons were a common trope that children would be familiar with. Using a Mayor or Governor would require a lot more explaining for children to understand his role and have more political undertones. It also kind of made the land of make believe a little more fantastical.