No, it is because onions (and potatoes) occupy this very weird middle ground in terms of perishability, the seasonality of the harvest, and the size of the domestic production and consumption market, which makes them uniquely vulnerable to market manipulation. A commodity has to be storable enough and/or regularly harvested enough to create a predictable futures market, but when inventory is small, highly seasonal, geographically constrained, and slowly and often unpredictably non-linearly degrading, then it's pretty easy for a big player to squeeze the market.

It is a lot harder to corner the market when you have very non-perishable or very perishable goods and/or much larger domestic and global markets with harvests that vary across geography and time, like wheat and frozen orange juice concentrate. Most goods traded via futures have those properties. Onions is right on the edge of even making sense as a futures commodity, and because of that, it's a recipe for manipulation.

And there were calls to regulate potatoes the same way in the fallout of the onion market manipulation, but those didn't pass, and in 1976 the potato market was almost cornered in the same way.

But sure, govment bad.