He doesn't play an NFL-style game. Denver used the college style "spread option" formation with Tebow that year and surprised and squeaked out several narrow wins against a lot of NFL defenses who weren't quite ready for it. Now those coaches have all watched the films and know what's coming when Tebow comes into a game. He isn't going to catch his opponents by surprise ever again.
Thus, he has to learn to play in a pro-style offense, running designed plays proven to work against NFL defenses. He has apparently struggled mightily with that task in practice.
Thanks.