Because it is the arab dhaqan. For example the prophet didnt eat the dab lizard but when he was asked about it he said it is halal but my people dont eat it
Hadith of the Prophet Muhammad (saws) in English and Arabic
sunnah.com
That is an example of the arab xeer prohibting something that islaam permits. But xeer prohibition does not refer to sin it just refers to taboo or something. This means that if someone from the prophet's people ate the lizard they would look at him weird and tell him off for it but the eater wouldnt get actual danbi
Now we look at somali dhaqan the transcripts mention. That cousin marriage was considered danbi, but this is the same way eating that lizard amongst qureysh would be called danbi. They would be looked at weird but they have not commited actual sin
Note this does not work the opposite way it cant make halaal what the sharci prohibited, so xeer is always secondary to sharci. But it is still considered