According to Takao Unno, the director of Pokémon Black Version 2 and Pokémon White Version 2, the protagonist in each of the core Pokémon games never speaks because the developers want the player to feel he or she is that main character. By keeping the main character silent, Unno hopes players develop strong bonds with the Pokémon and trainers they come across on their journey.
“As you know, the protagonist in the game never speaks. The idea is to make it feel like the player is that character as much as possible. By doing this we’re also hoping that the player has a better bond with the Pokémon they encounter, as well as the people they meet inside the game.”
-Takao Unno, director, Pokémon Black 2 and White 2