I like the Tarnished Archeology theory that there was a war between Leyndell and Mt. Gelmir and that Messmer fought on the side of Mt. Gelmir/Serpent god and as such was ostracised from Erdtree side. His throne is missing from the Morgott arena and he has snakes about him as well.
But I don't see any other lore slingers validating this theory so I am unsure. It would be cool nonetheless .
I would love to see a source for this. Even if this is true, it's probably possible to circumvent regulations like this during war time. If elections can be suspended, opposition political parties can be banned then this is like possible as well.