Because he's being too stubborn for his own good, refusing to switch over to halfback or tight end like numerous teams have asked him to do.
Well, that's the whole problem. Tebow needs to realize that he needs to be on an NFL field making plays for all to see, regardless of position. Theoretical discussions about what he might eventually do at QB don't help him.
Kordell Stewart was no great QB, either, but he lasted quite a few years in the league because he was willing to start out primarily as a WR and on special teams.
Tebow has fought to play QB since his pee wee days. He went to the worst team in his county so he could play QB in high school. They won the state championship his senior year.
He had a winning record in Denver including a big playoff win. A recent Forbes poll found him the most influential person in sports and that's why he wants to play QB. He's not doing this for himself or because he's stubborn. He could be making a lot more money in baseball with a lot less grief.
If he is refusing to take a non-QB position, then his own pride is his real enemy. It’s not for the player to tell the team how he’s going to be used.