Eggplant (aubergine in Europe) as an American. They're extremely common--just about every supermarket sells them, even the smaller, less fancy ones from when I was a child. But I've rarely seen them used, never as a side vegetable, never in salads, never a sandwich ingredient (which between those covers 99% of the other vegetables). No one ever talks about eating eggplant except maybe as a punchline or if you were a vegetarian.
Sure, there's a market for them but it doesn't justify being literally everywhere.