The princess told me that, at the birth of Equestria the unicorns insisted on a magocratic rule. The other factions; the militaristic Pegasi, and the democratic Earth ponies didn’t understand.

The other tribes foolishly agreed, which meant that for several hundreds of years, only the most magically gifted ponies could become the leaders of Equestria. In fact, that is how princesses Celestia and Luna were able to take power without endless bloodshed.

Nowdays, we have an open system. Anypony that wants could try and themselves elected leader of Equestria. Ponies are content to leave it to the royal sisters, however.