Two things come to mind: 1) practicality and 2) cost/convenience.
Consider the problem with "porch pirate" thefts when delivered articles are left on someone's porch unattended, often because no one is home. A drone can't even put your package on the porch, out of the weather, so unless someone is home and notified of the incoming delivery, it will sit in the yard (or on the roof
). This seems highly impractical to me.
These drones will have far less payload capacity than the traditional van/truck. Even if the charges for drone delivered items is the same as for ground-based, it would take a steady stream of drone traffic to approach a single day's ground delivery currently in my neighborhood. Imagine the noise. How cost effective is it to dispatch a drone to deliver, say, a pizza or a pair of tube socks? This is literally pie-in-the-sky for delivery services and except for specialized items, I don't see it being commercially successful.