My guess is they are doing this to either pick the most profitable route for themselves (i.e. high density communities, college dorms, apartment complex, small items that doesn't take much space or weight, low cost items that they don't need to worry about thieves), and let other carriers deal with the less profitable ones.
Another possibility is union busting. It is cheaper to contract the "shared economy" than to pay the union and the middleman.
They are building up the Fresh food delivery service, so might as well deliver the non perishable items.