Basically, our physical dimensions dictate the heights of things we find comfortable. Toast tends to fall from the surfaces at a slow speed. The slow speed drop from the surface means a particular rate of spin. Since the height of tables, counters, carried plates is all pretty close, the toast can't rotate far enough in that height to avoid landing on the buttered side.
Well, at least I thought the analysis was interesting but I'm a nerdy person. http://io9.com/5867322/an-experiment-that-solves-the-worlds-most-important-question-how-to-keep-toast-from-landing-buttered-side-down