Why are commanders limited to creatures/planeswalkers?

I'm working on a deck that intends to draw the entire deck to win via [[Thoracle]] the hard way. However, it got me thinking, why are we restricted to legendary creatures and a select few other things in the Command Zone? If I could, I'd 100% build this around [[Bident of Thassa]] or [[Jace, Weilder of Mysteries]] in the CZ, but instead, I'm stuck with worse options such as [[Tomorrow, Azami's Familiar]], or [[Grazilaxx, Illithid Scholar]], because they're creatures. I know you can Rule 0 it based on the playgroup, but mechanically, why not allow ANY legendary permanent in the CZ, whether it be planeswalker, enchantment, Artifact, or even a land?