They were probably going for something that would be light and airy and not so grim and formal as some of the other presidents' portraits are.
They also wanted Black artists. They weren't very picky about anything else. I guess they got what they wanted.
I doubt Obama chose. Most things come through underlings investigating and presenting options. I would HOPE that the beheaded whites weren’t part of what was presented to him. The fact that the painter is famous enough to use a factory approach was probably appealing. The fact that he was black was essential. Since Obama has presented himself in traditional portraits, I’d guess that the shock effect was a big part of the appeal. The “I” President has guaranteed that he is, once again, the center of attention. And you’re quite right. He’d never choose anyone who could see who he really was.