In the main series, Shiny Pokémon were first introduced in Generation
II. This was likely to take fullest advantage of the Game Boy Color,
which enhanced Gold and Silver, and was required for Pokémon Crystal
This means Gamefreak wanted to make the most of the Gameboy colour, because back then, it was all black and white.
(Fun Fact: The term "Shiny Pokémon" was created by fans to refer to the sparkling sound effect and animation made at the start of an encounter with one in the games. Eventually, this term fell into official usage in Generation IV.) -Bulbapedia
(But note, Bulbapedia doesn't have everything, so never stop asking questions!)