Cactus is cheap, delicious, and can be found in your local Hispanic foods mart. Prickly pears are high in fiber, and only about 40kcal/100grams — Sprinkle a little bit of Tajín on them and you've got a guilt-free snack.
I suppose it will become more common in the future, but the main reason for that will probably be due to the growing Hispanic population in the US.