Can put lot of reasons :
Thoses potatoes will kill the economy , cost it cost nothing , make people supra strong and cost less to produce/use due to size, hence the old potatoes become useless.
Hence it will make others people jealous , so villagers need to defend production with probably military means , counter espionnage , a force against invasions from others countries.
Add to that thoses potatoes give a lot of nutrient , if it continue to increase it can even become "toxic" cause too much to digest . Overnutrition for exemple , Mineral poisoning etc cause too much of one element. Think of it like yourself eating 20 eggs per meal , Hard to digest , Hard to transform , and it will probably kill you in the end.
For nature , strong species supplant the others , so thinks about all thoses "super flora" let loose , with a little bit of evolution it could make a massive genocide of others plants