1.) His mechanics are still bad as a QB. His throwing motion and more are big risk factors and why the Broncos barely let him throw as much.
2.) He can't pass well in the pocket. Unlike a RGIII , Newton or Kaepernick , Tebow was pretty bad in the pocket as QB. It wasn't til the Broncos started letting him roll out and get outta the pocket that he could throw a bit. Teams like New England , KC and others would then use LB's on the outside to force Tebow later to stay inside the pocket where he failed.
3.) Can't pass his team back into a game. Once the Broncos got behind against the Patriots and other teams , Tebow struggled to bring his team back. Teams would put 8 in the box and dare Tebow to throw. He couldn't. His final game as a Bronco summed up how bad it was for Tebow. The Chiefs were ahead 7-3 winning , and put 8 in the box and dared Tebow to throw. He couldn't do it.