Why won't Pokémon Crystal play on an original Game Boy?

Gold and Silver were hybrids: Game Boy cartridges with color enhancements, playable on the original 1989 hardware. Crystal broke the line. It shipped on a Game Boy Color cartridge and was designed for the Color alone, making it the only Generation II core game that can be played on neither the original Game Boy nor its revisions, and the first core Pokémon game incompatible with the Super Game Boy as well.

The refusal is built into the object. Boot Crystal in an older machine and the cartridge itself delivers the verdict on screen: this Game Pak is designed only for use on the Game Boy Color.

The break made sense by December 2000. Crystal's additions, animated sprites for every Pokémon, the Battle Tower, and in Japan the Mobile System GB online features, were built for the newer machine, and Nintendo was ready to let the eleven-year-old original go. Crystal is the hinge cartridge of the handheld line: everything before it runs on a 1989 Game Boy, and nothing after it pretends to.

For the shelf, it is a useful teaching object: two cartridges that look like siblings, Gold and Crystal, belong to two different hardware generations.