I'm just throwing this "out there" of my own accord. I have no evidence that anyone involved in STAR TREK was thinking this, but Romulas may well have started out Vulcan-y, but may have been terraformed later, for any number of reasons. Romulas might've been like Mars, in having all of its ice locked up, just beneath the surface and when the Romulans terraformed it to make it hotter, all of that water melted out to form seas and all that. Though it's never been stated, I've always assumed water to be quite the commodity on Vulcan. When the Romulans took flight, this life-giving resource may have been under Vulcanian control ... there may have even been "status" relating to this, an attitude the Romulans held onto. Romulas may also be strategically placed, from a Romulan perspective. Claiming this specific planet may have been ideal as a hub to branch out from, as they conquered other territory.