But the Empire is not a democracy. The people have gotten used to the fact that who becomes emperor isn't their problem, unless it actually affects them negatively. And as I remember, the Reman Empire didn't care when the Potenate took over, and they didn't care when Sanguine lived in the Palace.
People prefer not to be ruled by someone of an alien culture. That is why when people have a chance, they will overthrow that alien in favor of a local. They will accept an alien emporer in good times, but in dire straits, that alien will be seen as that. The Empire is in dire straits, they will not accept an alien. The potenate resided over good times and was an important figure for three generations, so he was already fairly Cyrodiilized, and by that time many Cyrodiil's population had Tsaeci blood. We don't know how they reacted when Sanguine lived in the palace. That was Reman the second right?
But Cyrodiil did not let Katariah's son rule, because he was Dunmer, even though he was the closest in blood to Tiber Septim.