I suspect lots of inbreeding. In this the Hebrews were revolutionary—with their anti-incest laws. The Egyptians, in contrast, were expected to marry their brother/sister (and had lots of defective children...).
Even today the least intelligent regional/racial populations have lots of 1st-cousin-marriage.
Upwards of 75% of Middle Eastern populations (except the Jews) marry 1st cousins....and are substantially less intelligent in IQ than other peoples.
That explains the British Monarchy.
I think that was only Egyptian royalty.