I am under the assumption that the dark jedi that first came to korriban and interacted with the sith species had created a hierarchy that placed Humans (force using) above the more primitive Sith (force or not force using, they're described though as being almost universally force using). The suppression and slavery of the sith species likely caused some variety of preferred breeding (force users over non force users, human and sith being encouraged more than pure sith, influential/high caste [priesthood] members of the sith race encouraged to breed, lower caste members discouraged or prevented from breeding.) The uplifting of the "pure" sith is recent, and has been correlated with higher than average (human) strength in the force.
It was alluded to in KOTOR 2 that the pure sith were not extinct, just laying in wait for a good opportunity to attack the republic/retake korriban.