It does seem to be a less and less common occurrence for players who starred in college and especially in the NFL to become head coaches. Some of the best coaches in the game went to small D3 schools, and for say Nick Saban, lowly MAC schools even!
While technical knowledge of the game is a must, I agree that coaching is more of a business/management position more so than the strength and/or speed that is required to be a great player. In many ways a MBA is more beneficial than a Heisman trophy.
It's a lot like MLB and other sports. The "stars" are too busy focusing on their own performance and getting pampered to pick the other parts of the game. The backups get to watch the coaches more, so they learn other facets of the game, not just their own position.