I think the difference is that we're still valuing marijuana at prohibition prices, which means the producers have enough money left over they can bear the price of a system like that and still make a hefty profit. With something like corn, it takes an acre of land to net as much money as you'd get from a couple ounces of weed. [1]

[1] http://smallbusiness.chron.com/profit-margin-raising-corn-34...