Over the past week, Popeyes has trended on globally, welcomed expansive crowds and sold out of just about every new chicken sandwich it has. But why is this particular sandwich such a big deal?

This is because of the familiarity of the product among customers

Explanation:

Because this is a restaurant set-up, there must have been a vigorous advertising of the product. In addition, due to customer satisfaction and reviews, it is possible that the restaurant received good reviews and recommendations for their chicken sandwich. Hence, the popularity of the restaurant and the high sales of sandwich.

I think because it is in competition with other restaurants and wants to be the best restaurant that has the "ultimate chicken sandwich" i think all the restaurants particularly have the same kind of chicken sandwich but add and take of stuff to make it great