The main reason for this engine placement relates to aerodynamics (airflow). Newer engines are more efficient, but they're larger in diameter. In short, they won't fit well on the rear of the fuselage. They are more efficient when located out on the wing where the airflow doesn't interact with the flow on the fuselage and tail.
(This is a simple explanation that doesn't consider things like center of thrust, engines pushing on the body vs. the wings pulling, ground clearance, maintenance, high- vs low-wing, cost, etc)