Here's why:
* Different Generations: Pokémon Crystal is a Generation II game (released in 2000), while Pokémon Ruby is a Generation III game (released in 2002).
* Incompatible Trading Systems: The trading systems between the two generations are fundamentally different. They use different communication protocols and data structures for Pokémon storage and transfer.
Therefore, direct trading between Pokémon Crystal and Pokémon Ruby is not possible.