This is actually an easy question to answer.
1. Mainstream video productions are not going to cast a black man as Satan. Everyone knows better than to do that. If you want to have an actor in the role of God, that's a great time to cast a black actor, e.g. Morgan Freeman. That's something that makes Americans feel good about racial progress. That's what we do here.
2. Needing an actor to play the role of Satan, the casting people working on the History Channel's "The Bible" picked Mehdi Ouzaani, a Moroccan actor. He was chosen because he's a fine actor and he looks like a person who would have lived in the region during Biblical times. Of course, he's not black. They wouldn't cast a black man as Satan!
3. Since Ouzaani isn't black, they didn't think about his resemblance to President Obama. People tend not to see cross-racial resemblance. If you were putting together a "Separated at Birth"-type book, can you think offhand of any black and white celebrities you'd pair?
4. They didn't notice, but when the film went up on television, with millions of viewers, and with particular shots showing Ouzaani's face in the shadows, some of them said "Hey, Satan looks like Obama!" That seemed funny/outrageous. It got tweeted. Soon everyone's talking about casting an Obama lookalike as Satan.
5. And that's how it happened. Or... maybe the Devil made them do it.