"In the 2000s (decade), onion prices were significantly more volatile than corn or oil prices. This volatility led the son of a farmer who initially lobbied for the ban to advocate a return to onion futures trading."

This is the important part -- ag futures are insurance for farmers. You don't have to get it, but it limits your downside when you do, which is extremely important in low-margin, capital-intensive industries like agriculture. Banning the insurance may prevent one-off manipulations and limit the upside captured by speculators, but it also makes the industry much more dangerous, particularly for smaller players.