I'll take a shot at a theory. Pardon the length.
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600,000 years in the past, Sargon's people were a peaceful race of telepaths exploring and colonizing their region of the galaxy. They settled on dozens of planets including Vulcan and Mintaka. Over the next 100,000 years, their mental abilities grew so powerful that they began to consider themselves gods. They could transfer their consciousness into living hosts (much like the Vulcan katra) or mechanical receptacles, they could control the minds of others or cause great pain. They even had telekinetic powers that allowed them to control machinery, direct energy like a weapon, and more.
These abilities resulted in devastating power struggles on Sargon's world. Billions were killed. The dominant faction of the planet, still embroiled in a war that looked like it would destroy all life on the planet, decided that drastic measures had to be taken to ensure survival of the species. They went to every colony world of the empire and forcibly removed the telepathic/telekinetic abilities of all of their people by altering their genetics. Those that refused or resisted were eliminated. The results of this genetic change was a pronounced brow ridge that belied major changes to the brains of the subjects that prevented them to use their abilities. But simply removing their abilities was not enough, since they still had the knowledge to reverse the changes. So all of their memories were wiped, and their cities and technology were destroyed completely.
These new people would start over again from scratch with no technology or history. They had become essentially the proto-Vulcans of Mintaka in terms of biology. The Mintakans, lacking environmental pressures to change, remained much the same as they were then, without telepathic powers. The Vulcans however, living on a harsh world, gradually started developing their mental abilities once again over the next 500,000 years. By the Time of the Awakening, they had some relatively limited telepathic abilities once again (touch telepathy, the ability to transfer a consciousness into another body). The brow ridges that had prevented their telepathy had withered away over the millenia as the Vulcans regained their powers.
The Vulcans of that era were no less prone to violence than their ancestors, and once again a terrible war engulfed the planet. Psionic Resonator weapons like The Stone of Gol were used by both factions in the war and killed millions by turning their own aggressive feelings against them. Both factions needed a countermeasure to the weapon and went about it in different ways. A leader named Surak arose in one faction who taught that the only way to defeat such a weapon was to purge all hostility and passion from one's mind and act only out of pure logic.
The other faction sought a more direct way to stop the telepathic weapons by turning to genetics. In ancient proto-Vulcans they found that the brains were wired differently in order to inhibit telepathy. They began altering their followers with the changes to the brains and the byproduct of the elevated brow ridges. Soon they had millions of soldiers, scientists, and leaders who were resistant to the effects of telepathic weapons without having to follow a philosophy of non-aggression. The tide of the war had turned and those who marched beneath the raptor's wings soon swept across the planet once again.
The two factions, no longer able to use psionic weapons against eachother, returned to conventional warfare. But with their aggressive tendencies and their resolve to fight to the death even when it was no longer logical gave the Raptor faction a decided advantage. The Followers of Surak were almost broken and decided to take drastic measures to preserve their way of life. They launched a nuclear first strike against the leadership of the Raptor faction, hoping that by cutting off the head the rest of the faction would become disorganized. They were correct. Leaderless, they were fought to a crushing defeat at the foot of Mt. Seleya. The Raptor faction, desperate and chaotic, decided death was preferable to surrender, and launched an all-out nuclear attack on their enemies. The Surakians responded in kind. Hundreds of millions of Vulcans were killed in the exchange.
The remnants of the Raptor faction, broken and defeated, were given the choice of becoming one with the Followers of Surak or face exile. Some remained on Vulcan, but the vast majority decided to take to the stars and find a new home. These people became the Romulans. The majority had the brow ridge and thus lacked telepathic abilities, but some retained their un-ridged Vulcan appearance and mental skills.
Two millenia later, the Romulans would return to plot their revenge on Vulcan and her allies. It was decided that in order to sew distrust of Vulcans amongst their allies and for infiltration purposes, the Vulcan-looking Romulans would take point on most operations. Ridged Romulans only operated behind-the-scenes or were forced to wear helmets that concealed their brow ridges. They wanted the other species of the Federation to question their Vulcan partner's allegiance.
After a while it became apparent that this strategy wasn't working, so ridge-headed Romulans once again became the dominant and most visible force in the fleet by the turn of the 24th century, as befitting their vastly greater population on the homeworld. The Vulcanoid Romulans, always resented for their appearance (which reminded the Romulans of their defeat) and latent telepathic abilities, were the subject of great bigotry on the homeworld. That bigotry evolved into out-and-out genocide and forced relocation on many occasions since they were no longer militarily necessary and protected from harm as such. Millions were exiled to Remus to oversee the mines, ostensibly "for their own protection" from the Romulan populace according to the government. Over the next 80 years they often interbred with the native Reman population, thus producing telepathic Reman/Romulan hybrids.
By the late 24th century a non-ridged Romulans was an extremely rare sight on Romulus itself, and one who openly displayed their telepathic abilities even less so. Many of the non-ridged Romulans who lived on Remus were killed in the uprising of Shinzon, so they may be close to extinction.
