I respectfully disagree with the above answer. Both Christians and Islamists enslaved Africans, in fact the first black slaves sold to Europeans were sold by Muslim merchants. The Koran forbids the enslavement of believers; many Muslims have interpreted this to mean that enslavement of non-believers is permissible. Muslims probably enslaved more Africans than did Christians.
The real appeal of Islam is its belief in the equality of all individuals. The Koran teaches that all are equal in the eyes of God. This equality was and has not been practiced by Christian Americans who have long been inclined to draw distinctions (rightfully or wrongfully) for any number of reasons, including race. It is the insistence on equality of all that makes Islam appealling to African Americans.
A major reason for this is that Islam has been seen as a religion that is less connected to racism than Christianity is.
African Americans have, of course, been subject to abuses at the hands of white Americans since the first blacks came to the North American mainland in 1619. In many cases, Christianity was used to justify that abuse. For example, Christianity was used to justify slavery and to encourage blacks to remain obedient to their masters.
By contrast, Islam did not have this connection. Although Muslims from North Africa were involved in slave trading, they were not the ones who are most connected to slavery in American history. Instead, Islam is seen as a religion that is practiced by many non-whites and is not connected to historical white supremacy. Therefore, some African Americans have argued that it is better suited for their community than Christianity is.