According to an article based on an interview with University of Nebraska-Lincoln professor Roger Mandingo, the reason McDonalds' infamous McRib sandwich is only available periodically, and for limited duration, is because the popularity of the sandwich drives up the prices of the ingredients:
And to this day, the McRib comes and goes from the McDonald's menu for reasons that have to do with its intense popularity and a national supply of pork trimmings that's typically a lot more limited than the supply of beef trimmings.
"If you suddenly start to buy a large amount of that material," said Mandigo, "the price starts to rise."
As the cost to McDonald's rises, the McRib tends to go out of circulation again. And then the same parts of a hog tend to flow back into the processing lines for Spam, Vienna sausages and other specialized products.
Does the release of the McRib really trigger a surge in pork trimming prices, and does the removal of the McRib from the menu correspond to the prices rising past a certain point?