Maybe bumpy heads were considered sexy for a time...........
It could've had nothing to do with the bumpy heads, the sex appeal. The bumpy-headed could have just happened to be the leaders, of higher rank than the average Romulan/Vulcan - whatever you want to call them right after they diverged. Or those with bumpy-heads by side effect could have been better warriors, or smarter, or more potent, or whatever. The bumpy heads could be a secondary side-effect of the primary reason for the sex appeal.
Or, in the days whe the bumpy heads were just starting, the proto-Romulans could have still been following the Vulcan tradition of arranged marriages, and sex appeal had nothing to do with it. The one or two who had the gene could have passed it on regardless and, in a limited population, those things can spread quickly. I recall something vaughly having to do with Rh-factors in a subset of German immigrants (or maybe it was Dutch?) that is a good corrollary for it, but I don't remember enough of the details to be helpful. Something from high school biology, at least.
Or, to be even more out there, it's possible that the bumpy headed subset existed on Vulcan before the scisim and banded together - perhaps they were a clan, or perhaps they were outcasts for being imperfect Vulcans, or however Vulcan society may have worked before Surak - and went on to become the Romulans, and so there was no choice in the matter. Perhaps, if this was the case, the bumpy headed were chosen for the very reason they looked different than your average Vulcan.
Though, to tie back to the T'Pol's father thread, I'd like to hear
that explination. You can cosmetic surgury a Romulan into a Vulcan all you want, whatever other differences there may be, but if T'Pol's father really
was a Romulan, genes don't lie.
If she was half-Romulan, she'd have half of the Romulan genes, including the bumpy-headed gene. Now, it might be a dominant/resessive thing, the bumpy head, but that would have to bank on the fact that her supposedly Romulan father was carrying that gene and was lucky enough to have it passed on to her - 1in4, if he was a de-bumped Romulan, 1in1 if he just so happened to be a bumpless Romulan....
Sorry for the bio lesson. Genetics and organic chem were always my fav in school....