It's the same reason you start Hicks over Collins. If you believe that Hicks won't make the mental mistakes that Collins will make, even if one of the players is a lot more physically talented, you go with the guy who makes less mistakes. The Cardinals right or wrong, don't want to train players on the field when they have veteran alternatives who can be consistent right now.
Remember Bobbie Massie's rookie year? His first eight games were as bad as anyone had ever seen, but his last eight games he was actually pretty solid. Arians came in and was so concerned about his inconsistency that he signed the corpse of Eric Winston to start over him. Winston was trash, but he wasn't as bad as Massie was over the first eight games of his rookie year.
It's a pretty reasonable idea.