[Secret Agent Man]

i would guess the first main reason is inbreeding.

[sagar]

Islam mandates inbreeding for marriages (but not for rapes). Here is the world inbreeding map. The darker the color, the more inbred the country.

[tbw2]

I read an analysis that another reason that cousin marriage is so high is because cousins are the only women a man can interact and escort with that he can also marry.
Hence, the only woman he could walk to the market or talk to in private that isn’t a sister, aunt or niece is a cousin. So he ends up being far more likely to marry her unless he’s rich enough to pick up a foreign bride.