I see it more about playing style than race.
If you're a "running" QB, you are incredible for 1-3 years. Then, you sustain enough hits to be average or bad. It's not sustainable. You're blazing all over the place in your 3rd game, everyone says you're amazing. In game 14, chasing a playoff spot, you aren't nearly as fast, and your shoulder has taken a shitload of hits, which ain't good for your throwing.
Everybody on Tebow's nuts... Remember how good he was for games 1-6? Then he lost 3 outta 4? Won against a weak PIT in OT, and sunk against Pats.
year TWO? Anytime they put him in at QB
year THREE? They let him play an entire game at QB against 4th tier scrubs being cut the next day, and he was missing open guys and throwing INTs. manning or brady would be getting 9 TDs, given a full game against guys that'll be on the practice squad tomorrow.
Running QBs should run 5 times a game, max. Then you can play for 12-14 years at that level. Can't build a franchise on a guy that gets pummeled 12 or 15 times a game. Cause they're gonna get slow, and those tackles hurt their legs AND their throwing.