Just a thought. Maybe if you do this again to include a self addressed envelope/postcard for feedback? I know it sounds a bit like fishing but if you explained that you spend quite a bit of time creating the sets but if they would rather have something else you would rather spend your time more productively. Most teachers understand time constraints.
FWIW your cards are wonderful. I tend to give the benefit of the doubt and think it is oversight by teachers because even if they can't personally use them they would be a lovely "re-gift" (ok I'm of the ilk that once I give something if someone doesn't want/like it then they can pass it on to someone who does).