At the moment, my student, Twilight Sparkle, is still learning how to be a Princess. Princess Cadance was for the most part uninvolved in rulership for many years, but now she and her husband oversee the Crystal Empire now that it is no longer trapped by King Sombra's magic.

In general, Princesses find their way to positions of power because they are most suited to them, and because a pony who does not want to help and care for their fellow ponies does not become a Princess.