It's the same reason you don't see tons of farms growing asparagus, brussels sprouts, etc. It's far easier to make money, and in a stable, dependable way, when you offer up the same few crops that everyone is used to, and that you can easily sell to wholesalers, who already have lined up purchasers. Doing 'niche' farming is always more time consuming and dangerous to the pocketbook, because you have to develop the chain to get them to the consumers, and you have to spend time building a consumer base. And, of course, that applies in all production, not just farming. It's why all the major car manufacturers keep spitting out nearly identical cars to one another, year after year, decade after decade. Sure, some people would love more choice, but it's 'safer' to build generically similar cars. They'll sell.