Could it be because they are a tiny fraction of the population of Black-Americans? I think they comprise 0.25% of Black-Americans.
Also, it's White Supremacists who instigate the vast majority of violence in America.
I can't recall the last time someone instigated violence
Here's what Wikipedia has to say about the Nation of Islam:
"The
Nation of Islam, abbreviated as
NOI, is an
African American political and
religious movement, founded in
Detroit, Michigan, United States, by
Wallace D. Fard Muhammad on July 4, 1930 Its stated goals are to improve the spiritual, mental, social, and economic condition of African Americans in the United States and all of humanity."
https://en.wikipedia.org/wiki/Nation_of_Islam
By the way, Wikipedia pegs their numbers at somewhere between 20,000 and 50,000 which means they would comprise somewhere between .1% and .25% of the population of African-Americans. That's an average of less than .18% of all Black-Americans.
Black-Americans have been heavily oppressed so you'd expect a certain fraction of them to join groups that would protect and aid them. This should be no surprise.
White Supremacists have no such claim of oppression and their numbers can only be explained by the continuance of illogical hate. Once hate gains a foothold it's very hard to dislodge.