So what's the list where they did happen. Terelle Pryor??? Even he got his chance to play QB in the NFL briefly. People love to pile on Bill Polian for his Lamar Jackson take (and rightly so) but I don't see the massive trend there. No idea who did and didn't run 40s and their combine and pro days I can give you plenty of names of plenty of athletic black college QBs where that didn't happen: Deshaun Watson, Patrick Mahomes, Dak Prescott, Russell Wilson, Kyler Murray, RG3, Cam Newton... I could go on.
I just find the notion ridiculous that him running a 40 is going to make NFL personnel want to move him to another position. QB is by far the most important position on an NFL roster... I don't care if he's a Lebron type athlete if he can throw the ball at an NFL level he's going to play QB. I think a far more likely reason for him to not run the 40 is he and his representation don't want to risk underperforming relative to the speed he's shown on tape.