1- on a smaller aeroplane, the trim/thrust effect will have more power than on a large long fuselage
2- the engine would have to be placed further back causing bigger trim problems. This rear positioning would be necessary to allow for easy removal and clearance from the rudders.
3- it would waste a large part of rear tailcone volume which is already there.
4- it would call for a higher tailfin as rudder space would be wasted. These jets live in tight hangars, so it would not be popular.