Oh, I don't want to dis the Romulans. They had a ton of potential. The show runners even got rid of those stupid looking uniforms when the Dominion started to make waves in Deep Sapce Nine.
And it wasn't so much that the Klingons and Romulans were similar, but that the Klingons were just way too popular, had all the assets the show could re-use and were more interesting to the writers to write. Remember, TNG and DS9 had no re-occuring Vulcan or Romulan character, but they did have a main Klingon character in the form of Worf. When Worf was brought onboard to DS9, Klingon stories shot up by about 250%. It's not hard to see why you would want to write for the Klingons when one of your main characters is a Klingon who has conflict with his own race. I can imagine using the Romulans would be a waste of time for the writers since, again, Klingons have cloaking devices anyways.