I agree. I think Braga and company just copped out. The Romulans had exposure and the name was recognizable, but they had very little development over the decades, as opposed to the Klingons, who IMO had been overexposed, and there was little that ENT did that added to them, except for the forehead origin, which I didn't have a problem with to be honest.
I think there was this reliance to go back to the most popular species, Klingons and Borg, by B & B, but in doing so they went to the well one too many times and actually made me tired of seeing them. Whereas the Romulans didn't seem to hold that much regard for them, though they were also a popular species. I figure that B & B just didn't get the Romulans, didn't find them interesting, and then projected their lack of interest onto the audience.
I liked the Xindi, I thought the Suliban were interesting, and it is true that you could do more with them than you could with established races like the Romulans. But to be honest, what exactly did they do with the Xindi or Suliban that couldn't have been done with the Romulans or Orions, for example? Its not like they wiped either race out, both were left standing by the end of the series. B & B didn't really get radical with either species, they played pretty safe with them, so why not go with established races that would excite a good portion of your fan base. I really enjoyed the Romulan episodes and the Orions from the Augment arc. I thought the Orion female episode was weak, but I still enjoyed seeing them. Same with the Andorians and Tellarites. I think ENT made a mistake by not using their time period to really show us more about Andorians, Tellarites, Orions, Romulans, and Earth. They did focus on Vulcans though and Klingons.