Peach is not a king, she is a monarch.
Kings and Queens are gender-based titles for crowned monarchs in a Feudal European-Style Monarchy (Just as
Sultan is the [I believe] Male head of a Islamic, and Nearly Sovereign, Territory).
Princesses and Princes are for Next-in-line for said crown, where as a "Crowned Prince" and "Crowded Princesses" are immediately next in line, taking precedence of non-crowned royalty.
besides, the toads are Peach's slave. I just wish there was a revolution like the French Revolution and the Mushroom Kingdom becomes a republic. (Let bowser invade Sarasaland so Daisy is forced to come to the Mushroom Kingdom and cause a rebellion duh.)
THe Japanese should make the Beanbean kingdom a democracy or something.
This would be difficult for the following reasons:
1) Politically: Establishing a democracy in a fictional land is a lot harder establishing a monarchy. Monarchy's have that "born-into-the-position" to fall back on, while a democracy (direct or republic) needs a constitution, elections, etc.
2) Popular Fiction: Then add on top of that, rescuing a social/political target has been in cultural-sphere since the beginning of story telling.
3) Commercially: A majority of gamers are male, and they-re are going to want to rescue a princess to fulfill the old, "rescue a damsel in distress" fantasy. (Think: Sleeping Beauty, Cinderella)
4) Historically: Revolutions (which this would require) are a bloody mess, and is outside of what's considered appropriate for target audience of Mario Games.
Also:
5) PEACH IS NOTHING LIKE STALIN OR HITLER. She has no secret police, she doesn't promote racial superiority, and i doubt she is a genus and/or Insane. There are no internment camps, no communes, no government regulated commerce, no firing squads killing people in the middle of the night to install fear - None of that!
When did we get into politics?
It's kind of hard to to get into "competent leader", without discussing Political Science & Philosophy.