Who knows the real reason that he's slipped under the radar, but my guess would be because of his versatility. For example, the people that loved him as Preach in the 1975 coming of age movie, Cooley High, may not of recognized that it was the same actor who played the college kid possessed by a 1940's gangster in the Blaxploitation classic, J.D.'s Revenge.
And those audiences may not have realized that Glynn Turman is also the same actor who played Mayor Clarence Royce in The Wire, or Alex Prince, Sr. in HBO's In Treatment, for which he won an Emmy Award. Other notable projects of his are the mini-series Centennial, the television movie Buffalo Soldiers and the feature films A Hero Ain't Nothing But A Sandwich, Five on the Black Hand Side, The Inkwell and many, many more.
In watching any Glynn Turman performance it's hard not to notice his energy, intensity and dedication to a role. And if a few other more famous actors could match his level of commitment, the movies of today would be a much more enjoyable experience.