Michael B Jordan's name was attached to this movie basically as soon as Trank became involved. Jordan starred in Trank's first major feature film. IMO, it's likely nothing more than good ol' fashioned hollywood nepotism rather than "oh, we looked far and wide and he's the perfect choice for Johnny Storm." I am doubtful that they seriously looked elsewhere for other actors of any color. Can I be sure? No, but it's not hard to put 2 and 2 together. And let's get real here, there's probably a thousand good looking male actors in Hollywood who could do a decent job in the role. But that's just the way it is in the industry.