With industrialization comes specialization.

You can’t reasonably expect farming families to be able to afford all the specialized equipment to be able to willy-nilly change the crops they grow and large corporations are just going to specialize based on economy of scale.

When all you need is a water buffalo and a shitton of labor you can diversify crops all you want but if you have to compete in a low-margin mechanized market you do whatever it takes to keep the farm going even if that means diversity gets thrown out the window in favor of cash crops.