Because a quadcopter is meant to be controlled remotely, while a traditional car isn't, so retrofitting it for remote control is likely time consuming and error

Because a quadcopter is meant to be controlled remotely, while a traditional car isn't, so retrofitting it for remote control is likely time consuming and error prone (ever watch one of the mythbusters episodes where they rig up a car for RC?). Self driving cars already have everything required to control the car built in. They just need to hijack the existing controls and hook it up to a RC system (or just tell it to drive to location using internal controls).