Because produce presents problems of both weight and volume, keeping bushels by weight is a good system.
If you go to an orchard and pay to pick your own, you will likely pay by volume. However, when that same orchard sells to a wholesaler by the truckload it makes far more sense to sell by weight.
Also, grain and produce can vary a great deal on size. Thus the amount of empty space in a bushel basket would vary greatly. A bushel basket of large kernels would make less wheat than a bushel basket of small kernels.
I forgot my upbringing. Dang it.