Actually there are two, one on each side. Same as every other cut on a beef. They pretty much always been restaurant item. There was a time when they, along flanks didnt sell well if at all in retail settings , they were seen a cheap cuts and with beef prices lower at time they didnt have the demand.I agree they aren't "new" but now they are a chic restaurant item. They are also pricey since there is only one per animal.
I also think that wide spread use of grills now common thing has change consumer habits.