Forbes Magazine just put out an article that highlights why "…the skies aren’t filled with delivery drones…” and it is an interesting read. The FAA recently approved UPS and Google’s Project Wing permissions for being an “air carrier”, but the zones they can delivery to is still relatively restricted. It is a Part 135 certification that allows these companies “air carrier” status. The main concern is the safety aspect of the drones themselves. They have to be without a shadow of a doubt “Type Certified” (the worthiness of an aircraft based on standards and practices of the FAA) for any of these drones to be allowed in the airspace.
Similar networks and companies already exist in highly populated areas, such as Zing. Drones are steadily increasing in reliability, however they aren’t truly held to the highest of standards. Especially considering that drones can hold up to 55lbs., you really want to make sure no preventable disasters sneak their way into production.