Facts. Quarterback is the only position in sports that's the de facto "leader" position. It's the most romanticized position in sports. The only one where if no player on your team really plays outstanding and you win a title, it's the one that gets the MVP. It's the position now where the MVP almost always goes to. In basketball, no team says "we don't have a franchise X so we need one".
A black QB HC combo being Walsh-Montana , Belichick-Brady, Reid-Mahomes esque would be
Black coaches is the next frontier, college and pros both, and it's gonna be that much harder because the coach's performance is less objective than the quarterback's.
With a quarterback, you can see exactly what they do out on the field and they can play their way into position. The big hurdle was getting Pop Warner and middle school / high school coaches to allow young black kids to play QB, once that happened they were able to just take over the ranks each step of the way up.
But with coaching, it's so much harder to "prove" yourself - first you have to get on a staff in the first place which is hard as hell, then you have to overcome internal barriers to work your way up, then you have to convince some other team that you deserve the head coach spot (advancing to the head spot internally hard as hell because if the last coach failed, they'll pin that on you too). Then even if you get that head coaching spot, they've proven that black coaches are fired quicker and for lesser failures than white coaches.
The play of black quarterbacks can lead the way and reduce racial stigmas. But with black coaches, no matter how well you can coach, it's hard as hell to "lead" the way because there are so many barriers that will exist regardless of your performance.
Then you get to black GM and it's even harder. And let's not even talk about black ownership. The path is long as hell.