Because he's a RIGHT TACKLE. Capitalization is important because no one seems to understand this. College Left Tackles can be moved to Right Tackle or Guard in the pro's if it doesn't work out for them at Left. But seldom, if ever, does someone play Right Tackle in college and make the leap to Left Tackle in the pros.
Oh, and by the way, he's looked like a pretty average RIGHT TACKLE in all his workouts.