Part of it was some of the fans were infatuated with Vad's abilities. After the UNC 2012 game, and Vad ran for a bazillion yards, the hype train officially left the station, and everyone wanted to play with the QB with a seemingly much higher ceiling. Every time Tevin did anything perceived to be less than what Vad could do, out came the whistles to change QBs.
It was only after the 2013 season (Vad's only season as a full time starter) that people began to realize and appreciate what Tevin did for us despite some of his physical shortcomings.