That could be true about event plants being more desirable. But those of us with extras of the same color would sometimes like to donate for those without so much luck. I remember it taking 6 months of breeding nothing but tulips between events to get that last color I was missing. Plus there are bred-only colors of roses and Christmas roses now. Perhaps if the donation page were links to the plant pages showing the plant in whatever stage it was when donated instead of just seed images, it could function as the adoption thread does now.
I don't know why it's there or why someone did it, but the seeds were picked up and then disappeared from the database, meaning trashed and not re-abandoned to the wild. I guess some people want to permanently get rid of plants they don't want so that no one else will have any of their plants but them.