To clarify what John Farley is saying, putting the engine on the wing spreads weight away from the fuselage which allows the wing to be less rigid, On the wing they can be stuffed out in front to provide a bending moment about the wing axis which again means you can make the wing less strong.
Probably the only reasons planes don't have them on the wing are:
1) you need a taller/heavier landing gear
2) you need a bigger tail to compensate for engine failure at low speed.