Exactly my first thought, it's because he wrote it, why would they ask someone who didn't write it to perform it?
That interaction might have something important the author experienced that we, as readers, can't visualise.
Also, a big part of this is that many times minorities don't want to reinforce a stereotype. There's a fine line to tread between one-off self-expression and the worry that the majority think that this is what you do all the time, or a label. I can see why the author doesn't want to be seen as "Derrius, the guy who raps."
It seemed like he actually wanted to and that he said yes. So the story is a little iffy from his side.
https://twitter.com/drquarles/status/692809125294706688 S/O means shout out so he was pretty happy about it to put it up on social media.
I was just super hesitant here because we didn't know the complete story and everyone else seemed to be piling on the author.