In order to win the Heisman as running back, I think you need to definitely fall in the 1600+ yards range, have 20+ touchdowns, play on a championship caliber team, have pre-season hype and make big plays in big games. Burkhead could get in the 1600 yard park, could have around 20 touchdowns (probably more at this pace), but didn't have the pre-season hype nor has he made many plays yet that I would characterize as traditional Heisman-esque plays.
Burkhead is a tough, grind it out back. He doesn't have great speed or power, but he has a really good combination of both. As another poster mentioned, he's just not that flashy of a player either.
And as recent history suggests, if you're not playing for a MNC, your chances are slim.
I think he has potential next year, though.