1) he's a know British TV Actor (tv actors find it hard to break into film, especially when they are british) I can say this, im british, he's a good actor but like a lot of british actors overlooked.
2) he's black and sadly most films want a white male to be the lead character, hollywood is still racist although perhaps not intentionally or maliciously, but rather is focus on profits and marketing demographics means it prone to cast white males as the lead.
3) he's not had a breakthrough role as a protagonist or antagonist yet, because of the reason above he's often cast in a support role rather than the lead. perhaps becoming James Bond would boast him (or hinder him)
4) he's relatively new to film scene, it takes time to build up a reputation, even in the UK i would not say he's a household name yet, most people say he's guy who did "luthor"