You will have interference drag due to interactions between the nacelle and the wing. This is minimized by having the engines forward or aft of the wing (Synthesis of subsonic airplane design, Torenbeek, 1976). Clearly the latter is not favourable for a number of other reasons.
It is probably a nice bonus to have as much of wing, flaps and fuel tanks out of the plane of the turbine discs as well.
I also recall reading somewhere that having the nacelles slung forward of the wing counters torsional flutter, but I have no reference on that and it is not my area of expertise. On first look it does make sense.