I'm a social studies teacher. The reason why the Tropic of Cancer and Tropic of Capricorn are at 23.5° North and South of the equator is because the tilt of the earth is such that the sun is directly overhead at the equinoxes. This only happens because the tilt of the Earth is 23.5°. Not sure why everyone else knew that, but that's why I did.
Edit: This sent me down a rabbit hole and apparently it changes. Currently the tilt is approx 23.44 and it's decreasing by 46.8 arcseconds per century.