If the planet Vulcan experienced numerous atomic wars then perhaps the ridges are common to Vulcanoids in general and it was the Vulcans who stayed behind who mutated and lost their ridges. This might also explain why Spock and other Vulcans have a high resistance to radiation, the Vulcans with low resistance did live long enough to reproduce.Who knows why they have ridges. There could have been mutation from atomic wars, genetic predisposition or interstellar travel.
Three thousand years is a long time to remain a high technology civilization. If the Vulcan had a series of large non-atomic wars every few centuries, then they might have periodically lost the technology to achieve warp flight, perhaps even space flight. The Vulcans at the P'Jem monastery could have been marooned for decades or centuries on occasion. Also later, under the teachings of Surak, the Vulcan could have turned inward culturally, the lack of the need to explore on Vulcans parts that we saw in Enterprise (from T'Pol) easily could be philosophical aspect of their "new" culture, rather than part of Vulcans natural physiological make up.
