It's the same thing with skirt steak, and even chicken wings. Wings used to be cheaper than dirt because nobody did anything with them. Back when I ran a small kitchen I remember them being not quite giveaway prices but pretty close.

But now that wing shacks are prevalent, and fajitas have become more popular, the price for those two items has gone up accordingly.

I'm sure there are tons of other examples, but those two come to mind.