It always puzzled me: Vulcans turned to logical non-emotionailty because, we've been told, the other option was passionate self-destruction. However, the Romulans didn't follow that path and they--right up until the Hobus supernova (and that wasn't self-inflicted)--seem to be doing just fine.
With all the ponding I receive here at the BBS, one more "heretic" thought won't make that much of a difference.
Your question is appropriate, but IMHO the wondering is due to retroactive continuity (and I refer to the Einstein Quote below) and illustrates that the error is within the retroactive continuity and not the original materials.
We get the information about Vulcan's past from Surak (or Spock's knowledge about him) in "The Savage Curtain". Apparently that has been recorded well enough so that Surak/Spock is able to communicate this information.
But there is no record of dissatisfied Vulcans deciding to abandon their home and colonize a new world - in the
far reaches of our galaxy (according to the time it takes a message from the Neutral Zone to reach SFHQ).
In "Balance of Terror" Spock has no idea where the Romulans come from, all he can offer is some colonization speculation.
In ancient Rome (…) defeated political opponents were often not executed but exiled to the far reaches of the Roman Empire. I believe the Romulans to be the offspring of such exiles, originally deported to the far reaches of space and Romulus (think Botany Bay, Australia and “Space Seed”), left with the basic means to survive and to build a new world of their own, but deliberately deprived of warp drive technology to prevent their return home (They only have impulse power according to Scotty in “Balance of Terror”).
Apparently, this took place mostly during the pre-Surak era where this crucial information was lost in war, thus Spock never had this information.
Such conditions (and being deprived of power and armies that could do battle and thin out the Vulcan population) would probably give birth to a different mentality and self-awareness, which could explain the apparent discrepany you noticed. YMMV.
Bob