Question on Mildew

So there has been a lot of talk around the 2018 Bordeaux vintage as those who were organic lost a lot of crop to Mildew. In general, Mildew is treated by Copper Sulphate, which I believe is Organic. https://www.mcgill.ca/oss/article/you-asked/what-bordeaux-mixture. IF this is the case, why would those practising organic not use this? Is it due to the concerns on the environment? Thanks