My theory is the Remans where native to Remus. When the more advanced Romulans arrived in their ships and began to colonise nearby Romulus the Remans were either conquered or became allies.
Given how Nemesis showed the Remans conducting mining operations and the Romulans doing the flying round the galaxy in ships and running the government i tend toward the belief that the Remans previous encounters with interstellar races hadnt gone well so they were suspicious when the Romulans first showed up. The Romulans needed minerals/ore from Remus for building space ships and stuff as it wasnt in high enough quantity on Romulus but they had didfficulty with the harsh Reman atmosphere.
So the two races struck a truce where teh Remans would mine the ore for the Romulans and the Romulans would defend against otehr interstellar races. This worked well when the Romulan star empire was in expansionist phases as the Remans woudl be held in high regard for providing necessary resources but not so well for when in isolationist phases when those resources werent needed and Remans would be looked down upon.
Thats just my theory based entirely upon what we see of teh Remans in Nemesis. I havent read any of the novels and dont consider them canon anyhow.