CJD develops in about 1/1,000,000 people per year globally, and the Fore people have a population of ~20,000. In theory, that suggests there should be an outbreak within 50 years of the society regularly consuming brain matter. Perhaps that number could be reduced somewhat by the infection rate, but the infection rate seems pretty high, to reach the level it did as fast as it did each person with Kuru needed to infect 1.6 people. Perhaps sporadic CJD isn't as sporadic as we assume, and it's tied to being in environments where others have had CJD? Another possibility is tied to the seeming link between genetics and the infection rate of Mad Cow Disease. Perhaps the population regularly went through boom-bust cycles, where everybody with 2 copies of the recessive gene that makes one more susceptible to transmission gets killed off, then the disease dies off for a long enough time for the carriers to re-introduce their recessive trait back into the gene pool.