The Fokker 50 is an excellent plane. It is based on a tried and tested airframe, the F-27, stretched to accommodate more passengers and modernized with newer engines and advanced avionics. I like the advanced engine controls which allow the pilots to set takeoff, flex, climb and cruise power by pushing on a few buttons in the center of the instrument panel.
The pilots who fly the plane for a living have nothing but praises for it. Passengers also feel it is a comfortable plane. So why did the ATRs outsell the Fokkers by a wide margin? That's because the ATR has a very lightweight airframe. The Fokker is built like a tank, yes, but the airlines have to pay higher takeoff and landing fees which are always based on gross weight. I guess a lot goes to the long gear legs of the F-50. The ATR has shorter, lighter gear which rest inside sponsons along the fuselage sides.
In the end, airlines felt that they could make more money on the lower operating costs of the ATRs.