This against a millenia-old genetic tie and interstellar migration that passed into faint memory and near-legend by the 2100's (Earth calendar) plus the fact that Spock sees Romulans as "Fallen Vulcans" who need to be brought to Surak.
Sometimes I think it's Spock who needs the attitude adjustment, not the Romulans.
If we're going by numbers, it's the Romulans who probably represent the dominant cultural trend in the Vulcan species. While Vulcan recovered from its near-terminal wars and fortified itself in its small corner of space against neighbours like the Andorians, Vulcan's offshoot colony on Romulus spent its two thousand years of independence creating a vast interstellar community. If said it elsewhere, but even leaving aside possible offshoots like the Rigellians aside, Surak's ethics are still big only among a minority of Vulcans. Trying to make converts to Surak's philosophy among Romulans who are culturally predisposed to a certain xenophobia and skepticism among Vulcann isn't a bad idea, but trying to make Romulans buy into the very historically contingent experiences of their planet-bound kin ... ?
Kamenor might be more right in saying that a certain amount of cross-pollination is all that can be hoped for, although I think you still need a movement for that kind of cultural embassage.
Why is Spock so big into the idea of Romulan-Vulcan unification? There are suggestions, from Taking Wing
, that the idea is fairly popular on Vulcan if not among the Romulans. Maybe Vulcans see it as their way of completing their civilization's recovery, of tidying things up.