Willy Staley explains why the McDonald's cycles the sandwich in and out of its menu – basic economics:

The theory that the McRib’s elusiveness is a direct result of the vagaries of the cash price for hog meat in the States is simple: in this thinking, the product is only introduced when pork prices are low enough to ensure McDonald’s can turn a profit on the product. The theory is especially convincing given the McRib's status as the only non-breakfast fast food pork item: why wouldn't there be a pork sandwich in every chain, if it were profitable?

James McWilliams questions whether McDonald's pork supplier treats its animals ethically.