Your assertion that no one is "getting a chance" is absolutely silly. Short QBs are on rosters, are able to play in practice, and preseason. Unless "getting a chance" means starting a full season, I don't know what to say to you.
Troy Smith: Won the Heisman, was 5'11, started 8 career games. Wasn't any good.
Case Keenum: 6'1, started 54 career games. The verdict? Mediocre starter/backup tier.
Seneca Wallace: 5'11, 25 games started, not very good.
Michael Vick: 6'0, 115 starts, got hurt, tantalizing but failed player.
Johnny Manziel: 6'0, started plenty of games, not good.
Rex Grossman: 6'1, not very good.
Ty Detmer: 6'0, 25 games started, not very good.
I can keep going. These guys all got their chances. They didn't do much of worth with it. That's why Russell and Drew are outliers.