Islam was founded by Arabs, not black people, and the civilization that originated from it has kind of a bad track record when it comes to treating black people as equals - some Islamic leaders during the first half of the 20th century were on good terms with the genocidal Fascist regimes of Europe, too.
Despite this, Islam - and several heretical and heterodox black supremacist spins on it - was quite popular among the more radical elements of the Civil Rights movement. Why Islam, and not one of the native religions of Africa (especially West Africa), one of the syncretistic religions of the Caribbean, or even one of the homegrown Christian denominations like the Ethiopian Orthodox Tewahedo Church?
It's kind of weird how people fighting against modern forms of slavery had a collective hard on for a faith whose leaders looked down on them as inferior for centuries.