Here's why:
* Riolu's evolution, Lucario, is not available in Explorers of Sky. The game's Pokédex only features Pokémon from generations 1-4, and Lucario was introduced in Generation 4.
* Evolution methods in Explorers of Sky are different. The game uses a system based on friendship and level rather than traditional evolution methods like trading or specific items.
If you're looking to play a game where you can evolve Riolu, you might consider other Pokémon Mystery Dungeon games like Pokémon Mystery Dungeon: Gates to Infinity or Pokémon Mystery Dungeon: Super Mystery Dungeon, which do feature Lucario and its evolution line.