The problem with Force Users in government is that the Force is non-utilitarian. Choosing the lesser of two evils isn't something Jedi, or really any light side oriented force user, can acceptably do. Such actions gradually taint and corrupt them. But politics, in a universe of limited resources, demands hard choices of this kind all the time. Consequently, putting force users in charge of the government for anything other than extreme emergencies (ex. if the Republic needs to fight an all out war with a Sith Empire) is just a bad idea.

There are lots of examples throughout Star Wars of non-force using characters getting away completely unscathed with the kinds of actions that would drive force users to the dark side. one of the most famous Legends examples involves the deaths of Palpatine. In RotJ Luke has to throw down his lightsaber. He can't kill Palpatine on the Death Star, doing so would doom him and everyone in the room knows it. But in Dark Empire, a furious Han Solo shoots the cloned Palpatine in the back, killing him for the last time, and it's nothing to him.

Bottom line: politics is a dirty business, and force users have to be pure.