High wings allow a shorter, stronger, fuselage-mounted undercarriage. With the fuselage closer to the ground it easier for the aircraft to carry its own air stairs, thus speeding up turnarounds and allowing use of less well equipped destinations.
The choice of turbofans may have been influenced by their relative quietness compared to turboprops around at the time. I seem to remember that its low noise level was a factor in its selection by operators using Orange County Airport in California.
http://articles.latimes.com/1985-02-..._1_flight-path