Against Thaumatocracy

Given that being capable of magic is better than being incapable, what are some reasons mages might not be in charge?

- There aren't enough of them
-- It requires innate talent which few have
-- It takes too long to learn
-- They keep dying
-- Numbers are limited by the need for some external resource. For example a mage has to bathe in dragon's blood or make a contract with a manticore
- There are too many of them. Being a mage isn't a big deal, and hiring some is less of one.
- Magic isn't that good
- The kinds of people who learn magic are bad at maintaining political power
-- They're too busy with magical studies
-- Magic is linked to certain characteristics such as lack of ambition or gullibility
-- Magic alters mages, for example removing the need for food and replacing it with an insatiable urge to visit places of power
-- Magical expenditure requires long periods of isolation or hibernation
- The kinds of people who learn magic don't want to maintain political power
-- Magical talent isn't hereditary. Ensuring your family will be passed over for some genius discovered in a village is distasteful.
- The kinds of people who learn magic can't maintain political power
-- The gods who give them power forbid it
-- The terms of their contracts forbid it
-- The dragons who rule the world forbid it, and they do check regularly
-- Magic circulates through the bonds between people, so that the more subordinates one has, the more his magic is divided among them - The rulers are capable of preventing it
-- They aren't mages, but they have magic
--- Divine sanction
--- The approval of the land itself
--- Contracts with manticores
--- A store of artifacts
--- Hereditary talents
--- They're the source of magic
-- They're above magic
--- Gods rule directly
--- Dragons/manticores/basilisks etc. rule directly
--- Aliens rule directly (they came to research magic)

There are a few reasons. As for how mages might become an oppressed minority, they can't, cut that out, your allegory stinks.