Why cant you use darkrai in Pokemon Battle Revolution?

You can't use Darkrai in Pokémon Battle Revolution because it wasn't released until after the game came out!

* Pokémon Battle Revolution was released in 2006.

* Darkrai was first introduced in Pokémon Diamond and Pearl in 2006, but wasn't available until later in the game's lifespan.

Therefore, there wasn't a way to get Darkrai within the timeframe of the game's development, and it wasn't included in the game's roster.