Run out of staples
The list of largest crops is not the same as the list of staples because the biggest crop is sugar cane, which has other applications besides rotting your teeth. While China grows a lot of all the major crops - appearing second or third where it does not dominate - and now produces the most potatoes, Brazil harvests the most sugar cane.
Agriculture is hardly less problematic than other industries, despite its ability to feed the world: there is now a sugar tax in the United Kingdom, in recognition of its harmful effects; Indonesia's cash crop is responsible for clearing forests and wheat may grow on land everywhere but it is no good to coeliacs and there is a growing market in gluten-free products. Only raise the spectre of the Irish potato famine to realise the vulnerability of potato farmers concentrating on one variety. The size and taste that makes it ideal for french fries still makes it a target for pests.
The popularity of a vegetable waxes and wanes if you are growing fad foods. It's too soon to tell if quianoa has the staying power of the sweet potato or will achieve the scale of maize or rice.
Turkey is strictly a thanksgiving thing according to one source, listing it as the least popular meat. ABC National's Country Breakfast informs us that chicken went from the least popular meat in the 1940's to "the kingmaker of today"