That's because the Ashkenazi have what's
called endogamy, which is the practice of marrying within the community.
Local populations became entrenched, eschewing intermarriage with other groups and adopting a cultural pattern of what researchers
call endogamy, the practice of marrying only within an ethnic or social group.
Not exact matches
There is actually a term for this
called «
Endogamy» which Wikipedia defines as, «The custom of marrying only within the limits of a local community, clan, or tribe.»