Don't really know, but might be because of cost of steel cars, vs. fleets of wooden
cars in inventory, built for the WWI war effort, coupled with the highly seasonal nature of grain shipping. Could have equaled a reluctance in investing in an inventory of dedicated cars that might remain idle for months. A related question might be...what were they hauling all those other dry powder products in at the same time....not open top hoppers subject to wind and contamination?
(For the period I am modeling, it means I don't need to buy any covered hoppers)