Isolating a small ethnic group leads to inbreeding problems; exogamy leads to hybrid vigor. There's a reason that purebred dogs end up with congenital hip dysplasia or the rest of the menagerie of problems they tend to have, and if you replicate that mating pattern among people, you end up with fumarase deficiency. Note that the leadership in that case is firmly rejecting any suggestion of exogamy. They are, after all, super-duper Chosen People.