I have two possible answers. Either Urban Meyer's Plan to Win spread offense was not ready for the tight end back in '06, or Burton has certain skills that Tebow does not possess with respect to the Urban Attack.
I read somewhere that Urban Meyer has been evolving his spread offense, and only recently adding in the tight end. He never used tight ends before he got to Florida, but we had a good one at the time and we gained Aaron Hernandez to help the coach develop his modified spread. On top of that, although Timmy will never be forgotten as the greatest Gator player in history, he may lack certain skills that Burton may bring to the table to help the spread where Tebow would not have fit.