This is because the inbred Islamofascists who are taking over Europe practice incest.
Islamonazis don’t like marrying outside of the family because they don’t like outsiders inheriting any of their stuff.
This is a built-in survivability mechanism of Islam. As the Votaries of Islam get stupider and stupider they are less and less likely to wake up from their Islamic Nightmare and and walk away from the gruesome practices it enforces on all its members.