From FatherofGray:
In Super Mario Bros. 3 it was established that the Mushroom Kingdom was an Oligarchy divided into seven lands, each with their own king. Each king had a magic scepter that gave them the power to rule over their own respective lands. Princess Peach was merely a figurehead and was ever so lightly implied to be the daughter of the King of the Grasslands. The Koopalings all stole the scepters, transformed the kings, and were to bring the scepters back to Bowser so that he could reign supreme over the entire Mushroom Kingdom with Princess Peach as his bride. His plan fails only because you as Mario/Luigi, stop him. In the newer games featuring the Koopalings we see them use the scepters, but we never see the Seven Kings again and Princess Peach is portrayed as the ultimate monarch. Not cool Nintendo. You can't just sweep an entire system of government and an important article of culture under the rug like that and expect us to forget about it. Retcons suck.(However, if Mario 3 was indeed all a stage play, the Seven Kings may have never even existed...)