Because it is not true that Arabs enslaved millions of Africans and.exported them.to slavery into Arabia if you read Zanzibar ots History and it People by W.H. Ingrams you will find that before 1917 Zanzibar that includes the whole Zenj Empire officially had a population of 5000 Arabs only and that includes old, women and children. How comes that they could heard about 15million Africans to slavery without any resistance or fight?
Recent research shows that the white people and especially the missioneries distorted the whole history of East Africa to appear that slavery by Arabs (Muslims) was so huge and rampant and that they were the only sarviours of the black people.
Now note thos photos you just took why you dont see an Arab owner beside his stock of slaves. Was it an Arab who would take those photos? Ask yourselves why in all history books there are not a single photograph that shows an Arab with the slaves except drawings? If course there were bad Arabs who enslaved people like Wazungu amd Wahindi but do not be fooled that it was an official trade as an establishment and organized thing.
In almost 500 years Oman was colonised by the Portuguess and Arabs were fighting for their freedom. Thanks to Imam Saif ibn Sultan Al Yaarubi who fought against and sunk 200 Portuguese Armadas at the Oman Coast the Omanis got their freedom. At that time the Portuguese were invading the East Coast of Africa, pillaging their lands and burning houses and rapping their females andhearding Africans like cattle taking them into slavery.
When the East Africans heard of the heroic victory of the Omanis they asked Saif Bin Sultan to come to their rescue where he did and went back to Oman and leaving East Africa to their own system of chiefdoms and small Sultanates like that of Kilwa and Tumbatu. So where do yo get this attrocities done by Arabs? Why doesn't Tanzania honour Abushir bin Salim or Moh'd Khalfan Rumaliza as heroes during Heroes Day of Uhuru celebrations? These Arabs led African revolts against the Germans who were doing the same thing Wareno did enslaving Africans?