They didn't.
The Republic of Venice had a thriving slave trade in Slavs (the word origin for "slave" comes from "Slav") captured by Tartars and sold in the Black Sea ports. Slavs in the eastern Adriatic were also captured and sold as slaves. Between 1414 and 1423, 10,000 eastern European slaves were sold on the Venetian market.
Large numbers of these white slaves were taken to North Africa and sold by the Venetians in Egypt.
Probably because they were a ready supply (provided by their African foes), cheap, and required little conquest.
Why did the Americas enslave Africans?
All of the above, plus the inherent ability of Africans to work well in the heat of the tropical climes of Central and South America and the Caribbean as well as the summers in the American South. Likewise the cotton gin as well played a huge role in exponentially increasing the demand for African slaves.
There are only a few vaguely uniques aspects of the African trade. First it was arguably the last and most recent large scale slave trade. Second it involved for the most part members of one distinct race enslaving members of another..albeit with help from their racial kin. Thirdly the distances involved in the trafficking. Fourth, the magnitude of it...I am not aware of another commercial slave trading arena involving these numbers.
One has to wonder what would have happened to these subjugated tribes had they not been diluted by slavery. The Ibo, Yoruba, and Coramamtee tribes are still today huge.