Going strictly by what we've seen in the shows Romulus has had very few colonies mentioned. I imagine that Romulus is seen as being the promised land due to it being the world chosen by their ancestors. It's possible that most of the other inhabited planets have a small Romulan presence, mostly military overseers using the indigenous population as slave labor.
Also, the Hobus supernova wouldn't have destroyed just Romulus but everything in between as well as in other directions. It's an expanding sphere. Spock may have used the red matter to stop it from entering Federation space. The rest of the RSE may be essentially depopulated.