The state is unpopular

Without elections and democracy the state has no authority... it is only because the state has a mandate from the people that it is able to claim any legitimacy. But the first-past-the-post (fptp) voting system tends to result in a two-party oligarchy which means the state doesn't have a true mandate. Democracy is a bulwark against the state becoming too powerful but if fptp is used then the people are unprotected. The state has not been given the permission of the people if only fptp has been used to have a true mandate the state must use a proportional system. All valid systems of democracy have the characteristic of proportionality otherwise there will be an incentive to vote tactically due to there being fewer candidates with a chance of success (which reduces choice). Without the use of pr the members of the state (the politicians) have not given the people sufficient opportunity to reject them. The people have not been given a veto if there is not proportional representation and so the state is invalid. There is no (valid) state if a form of proportional representation has not been used.