'Straight Outta Compton' is the tale of rap legends the NWA, and there was nobody more important to the NWA than Dr. Dre. Dr. Dre would probably have loved to play himself on film, but as he's now too old to play himself as a teenager or a man in his early 20s, someone else was required. Dre knew exactly who he wanted to play himself - Michael B. Jordan.
Jordan was flattered, and would have loved to say yes, but he'd just agreed to deal to appear in 'The Fantastic Four,' which was a box-office disaster. By contrast, 'Straight Outta Compton' featuring Corey Hawkins as Dr. Dre, brought in $160m. The 'Fantastic Four' reboot didn't even manage a third of that amount. Given the choice again, Jordan would probably have made his apologies to the 'Fantastic Four' crew and gone with Dre instead.