Rousseau is a French name. And things of Rousseau are -- what? Rousseauan? Or Rousseauian? The first makes more sense to me. Where does that i spring from? But many people do use the latter. In fact it's probably the more common usage of the two.
And then there is Canada. Something belonging to Canada is -- CanAYdian. Why? Why isn't the adjective of CA-na-da something that is CA-na-dan? Or, if you are 'Rousseauian', CA-na-Di-an? But no, the adjective is quite distinct, with a vowel change and the accent on the antepenult.
In that way, 'Canadian' is not much different from 'Floridian'. FLOR-i-da should give us FLOR-i-dan for the adjective. But instead we get an emphasis shift (to the second syllable) and an extra I near the end.
Who determines these things? They didn't ask me!