Why does the McRib appear and disappear at seemingly random intervals? An excellent post at The Awl has a plausible answer:

The McRib’s unique aspects and impermanence, many of us believe, make it seem a likely candidate for being a sort of arbitrage strategy on McDonald’s part….

If you can demonstrate that McDonald’s only introduces the sandwich when pork prices are lower than usual, then you’re but a couple logical steps from concluding that McDonald’s is essentially exploiting a market imbalance between what normal food producers are willing to pay for hog meat at certain times of the year, and what Americans are willing to pay for it once it is processed, molded into illogically anatomical shapes, and slathered in HFCS-rich BBQ sauce.

…The blue line is the price of hogs in America over the last decade, and the black lines represent approximate times when McDonald’s has reintroduced the McRib, nationwide or taken it on an almost-nationwide “Farewell Tour” (McD’s has been promising to get rid of the product for years now).