Mainly lifted from Spock's World, but generally uncontradicted with the rest of Vulcan-related canon: in Vulcan's medieval period there began a concerted effort to elicit nascent psionic abilities in Vulcans, both through legitimate marriage contracts and the stealing of psi-enabled people for enforced breeding. It was that propagation that led to the greater telepathic &etc powers amongst the general population. Primitive, but effective.