He had his chance years ago, as you have mentioned constantly. And the Vikings, the people who watch Jackson every single day, thought he was not the answer at QB. Those same people watch Jackson every day in practice, and think he's not the answer at QB.
As far as not getting it done, have you not been paying attention to what I've been saying all year? You know, about not having his receivers healthy, last week not having Peterson, having a coach everyone seemed to dislike, etc.? Since Rice has been back and Childress has been gone, the Vikings have won 2 games in a row.
Perhaps the problem isn't Favre, but the loss of their best receiver and a coach no one liked?