Well...it should go to defensive players more, considering the best player in college football does not play offense every single year. And that is a fact. I'm really not sure why there is such an offensive bias, either. Defense is half of the game. And judging from the majority of the championship winners over the years, it is more than half of the game, considering the dominant defensive teams with "capable" offenses win more often than teams who have it the other way around.
People are just so intrigued with touchdowns, yards, and stats over all. Good coverages, lights out hits, forcing a QB into getting rid of the ball quicker than he wants, imposing your will. None of it shows up on the stat sheet. A defensive player has to be either extremely versatile (Charles Woodson), or has to dominate in a manner that is record breaking (such as sacks, ints, etc.) in order to even garner mention, let alone have a decent chance at winning a heisman. I don't agree with it, because I do not believe that the heisman trophy truly signifies the best player in college football with the way it is currently given. But it's just the way it is.