I think that the answer to the OP's question lies in the fact that airliners are optimized for a relatively narrow high-speed range already. In contrast, high-performance gliders need to be optimized for 2 distinct speed ranges -- min sink speed for thermaling, and max range speed for cruising between thermals. While the difference may be "only" 30-50 knots, it is a significant percentage increase. Therefore, the use of negative or reflex flaps makes sense when the basic wing has to be efficient at the lower thermaling speed (positive flaps would add drag; and leading edge devices would add drag, weight, complexity, and cost).