For most farmers corn and soybeans are the most profitable crops. They are easy to farm in large quantities, and they store well. Most crops that you think of as more profitable are worth more, but not always profitable - either because so much labor is needed that you cannot scale as far, or because they spoil so fast that you can't be sure of selling your entire harvest before it rots (Corn can be stored for a few years if you need to, lettuce is not edible after a week). Also there is the demand problem - even if there is more profit in some other crop that doesn't mean there is enough demand to sustain adding another farmer without prices collapsing.