I am German, so I thought veeeery long whether to post this or not... Before I do, let me get one thing straight: I don't want to hurt someone's feelings with this post, I am just trying to be as objective as I can based on the things I've observed the last 20 years, either by playing lots of sports (soccer, basketball, tennis) or by watching even more sports (just about everything...)
When all is said and done, I am trying to give you an answer why black QBs aren't as successful in football as white QBs. For me it's logical but I'm of course open to discussion. I believe that black people do have superior athletic abilities compared to white people. This is a general statement and there are exceptions of course but as a whole I think this statement is valid. I think it's the same with QBs: black QBs are GENERALLY athletically superior to white QBs. And that's where the trouble begins: because of that black QBs rely more on their athleticism than on their brain. They are very successful in high school, modestly successful in college but hardly successful in the pros. They don't have to think about finding the open man in high school because it's already there. And if not, they just outrun anybody else. This is more difficult to do in college and it's almost impossible in the pros. But if you do that from the very beginning of your career, it's very difficult to adapt to using the brain more often at the later stages.
Don't know if this works for you or if it serves as a starting point... Again, I'd like to say that I am no racist at all. I've made friends with people from lots of places in the world over the years, one of my best friend is gay, another good friend is black. So, please feel free to comment but keep in mind: I'm being as neutral as it gets.