Because he had Michel, White, Burkhead and Bolden ahead of him. Just like White had Blount, Ridley, Vereen and Bolden ahead of him during his rookie year.
We could really break it down and look at the fact that Burkhead and Bolden have both played on coverage units so they also bring that versatility to the roster. I could be wrong, but I don't believe that Harris has that versatility, does he?