One of the main criteria that determines where an engine will go is the size of the engine! You would have a hard time squeezing an engine of suitable size under the wing of a Citation or Learjet. Consequently, bolting them on the back end is the next best option. Problem with bolting engines on the back though, is that you then have to beef up the rear end sufficiently to hold said engine (and its resultant thrust) in place. The underwing area is an easier area to build in the required strength to support a big fan engine.