Why does it matter? Whether you want to believe it or not, on a local level there was a mindset that blacks didn't have the mental capacity to be quarterbacks. Because of that, young black players were often guided to other positions. If they expressed interest in the position as they got older, they'd have to compete with players who had been quarterbacks their entire lives.

Quarterbacks on young teams are often sons of coaches, and there's historically been a dearth of black coaches because of the same thought process that impacted players.

There were never more than 12 black quarterbacks in the NFL in any season before 2000.

Ask again why it matters.