Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?

I think model airplain engines are very cost sensitive. Another poster
said reed valves provide low end torque but I don't think that model
airplane engines need low end torque. So my guess is they are not needed.