The Romulans are supposedly an "off-shoot" of the Vulcan race. They rejected Surak's teachings and headed out into space to find a new home.
Anyway, one has to wonder why they would have colonized Romulus, which seems to be an Earth-like planet that has very little in common with Vulcan's dry, hot deserts. It seems unusual that a species would chose to colonize a world that is so unlike their home.
Thoughts?
Your question makes two assumptions:
1) That Vulcan is a desert planet.
2) That Romulus is a lusher, greener more Earth like planet.
Thus you believe that Romulus was different from Vulcan in the time when Romulus was settled.
But planet Earth has many deserts and many other arid regions as well as lush, green regions, as well as being mostly ocean. Maybe both Vulcan and Romulus have many deserts and many other arid regions as well as lush, green regions, as well as being mostly ocean.
And maybe Romulus was different from Vulcan in ways that the original Vulcan colonists liked.
Or maybe Romulus was different from Vulcan in ways that the original Vulcan colonists didn't like, but they settled on Romulus because it was similar to Vulcan in ways that were absolutely vital for Vulcan biology.
Spock says that Romulans are probably descended from ancient Vulcans who had a warlike colonizing era. Presumably Spock is talking about a warlike period of colonizing other planets around other stars. Otherwise there wouldn't be any way for the ancestors of the Romulans to colonize Romulus.
Since Vulcan is only 16 light years from Earth, all Vulcan habitable planets orbiting the stars near Earth should be inhabited by Vulcans, coexisting with any native intelligent species - unless the savage warlike Vulcans exterminated the native intelligent beings.
But planet after planet habitable for Humans in the
Star Trek universe either has no native intelligent life, or has colonies of Earth Humans who settled there after Cochrane discovered warp drive, or has intelligent life different from either Vulcans or Earth Humans, or has native intelligent life that looks like Earth Humans but was living there long before Zefram Cochrane discovered warp drive. But no native Vulcan like people.
Either the Vulcans settled those planets many thousands of years ago but their Vulcan colonists died out or were exterminated long ago, or else the Vulcans never settled on those planets, although they did settle Romulus and some other planets.
Maybe the Vulcans settled only a tiny minority of the planets that are habitable for Earth Humans, because only a tiny minority of those planets are also habitable for Vulcans.
Amok Time:
KIRK: It's lovely. I wish the breeze were cooler.
MCCOY: Yeah. Hot as Vulcan. Now I understand what that phrase means.
KIRK: The atmosphere is thinner than Earth.
MCCOY: In this climate? If the heat doesn't get you, the thin air will. You can't do it!
MCCOY: Is this Vulcan chivalry? The air's too hot and thin for Kirk. He's not used to it.
In "The Deadly Years" when Spock is weakened by accelerated aging:
SPOCK: I have a question for the doctor. (Kirk leaves) Doctor, the ship's temperature is increasingly uncomfortable for me. I've adjusted the environment in my quarters to one hundred twenty five degrees, which is at least tolerable. However, I
MCCOY: Well, I see I'm not going to be making any house calls on you.
SPOCK: I wondered if perhaps there was something which could lower my sensitivity to cold.
MCCOY: I'm not a magician, Spock, just an old country doctor.
SPOCK: Yes. As I always suspected. (leaves)
This certainly indicates that Vulcans are comfortable at temperatures that are uncomfortably and dangerously hot for Humans and rather rare in most regions of Earth.
Humans whose ancestor adapted to living in the high plateaus of South America and Tibet find the denser air at lower altitudes distressing. It is quite possible that Vulcans can't live for long in the dense air of most regions of Earth. Healthy Vulcans can tolerate Earthly temperatures and air pressures but sick, weak, elderly or young Vulcans would be killed by prolonged exposure to them and so Vulcans can not settle on planets with temperatures and pressures that are too much like Earth's.
So the ancient Vulcans skipped over many planets like Earth and only settled on the rare planets with the right conditions for Vulcans, including Romulus, no matter how different Romulus was in other ways from Vulcan.