The simple answer here is because of slavery.
Answer:
In the 11th century CE the Awlil mines were in the hands of Takrur, but it would be the Mali Empire (1240-1645 CE), with its capital at Niani, that dominated the sub-Saharan salt trade following the collapse of the Ghana Empire.
It allowed for slaves to be counted as people halfway to please the abolitionists, but didn't allow them to be citizens to please the pro-slavery side.
