When Balance of Terror was written, the idea for the Romulan Neutral Zone was that they were confined to their own Solar System like the Kzinti in Larry Niven's stories. That's what Kirk meant when he said "The neutral zone separates them from the rest of the Galaxy". Obviously, they changed their minds later on when they wanted to use the Romulans more. That's all there is to it.
Also, Scotty's line about their power being impulse only is again part of the changed premise. In BoT "Impulse" was a form of FTL that was just less powerful than Warp, it didn't mean "Sublight" just yet.
Is there a quote that mentions the writers/producers intended the Romulan Empire to be limited to two planets? Hard to be a Star Empire if you only have one star.