Perhaps it's just like in Ancient Rome? Some of the Senators are genuinely elected politicians from the population, and others are "Nobility" who more or less inherit their positions. Romulan "Nobles" would probably be direct descendants of any Vulcan higher-ups who were exiled or command staff of the original colony ships.
It seems like the similarities between Romans and Romulans are not very well developed though. The Roman Empire was not xenophobic, they would often make people they conquered citizens and would try to improve infrastructure of areas they conquered. The Romulans seem to have no desires to do those things. Plus Romulan society seems more set up more like a modern totalitarian state, rather then the old autocratic state Rome used to be. I don't the Roman Empire had a secret police unit, for example, that is a more modern invention.
That's why Romulan society comes off as contradictory, they have a seemly totalitarian state, yet it has an seemly elected legislative body.
The Wormhole wrote:
How did it work in Rome? Since Romulans are basically space Romans, their governments are likely identical.
The Roman Senate lasted over a thousand years, how much power it wielded depended on which era of Roman history you want to refer to. Calling the Romulans "Space Romans" doesn't help define them very much, because there are different eras of Roman history and their society is very different during these different eras.