Mostly it's decided by who and how my protagonist is oppressed. Is there a single shot-caller who can be toppled or is it a complex network established by need/skill? Is the society established and entrenched, or is it developing? Am I building a fantasy world on top of a real one?
That last question might seem silly, but I write Urban Fantasy where the local tattoo parlor is a front for a battalion of Berserkers and a leather-daddy at the gay bar is an archangel. If I want the reader to believe the story is happening in the USA then I have to use the existing government of our republic; unless I explain the authority relevant to my protagonist is not the human government but a representative body of superpowers in which there are committees and a chairperson. Human governments of all varieties are a minor subset of the greater authority...which is what I did in my Immortal Spy UF series. In short, if writing contemporary fantasy using the real world as the backdrop, you can either leverage existing authority or you need to offer a reason for why/how your characters are bypassing it.
My High Fantasy works are more diverse in that the societies being changed vary in population, environment, history, traditions, and internal vs external strife. A semi-isolated civilization where there are rumors and vagaries of hostile neighbors yet everything within the society is strictly controlled lends itself to a monarchy. However, a nomadic society struggling to survive extreme scarcity where the nomads are not the top of the food chain is more suited to tribal democracies where leadership is decided by those who are led.
Messing with authority is crucial to my stories, so I like to use different types of government for my protagonists to rebel against.
