> Lettuce and Tomatoes don't keep well, which adds more seasonal variability
The more durable a produce is, the easier it is to transport over a large area, which will always smoothen price fluctuations. This alone could explain why lettuce and tomatoes have higher volatility than onions, which have higher volatility than potatoes, which have higher volatility than corn and weat.
At least that's the null hypothesis that the hypothetical effect of futures should be compared to.
I assert that onions and potatoes have roughly the same durability and ease of transport.
I know nothing about logistics of the food supply chain and how they preserve food at scale, but at least at home I'd say that potatoes are close to twice as durable as onions.