In an interview with 4Gamer (and translated by Siliconera), Pokémon series art director Ken Sugimori said that the reason that downloadable content such as new Pokémon characters will likely not be part of current and future Pokémon games is because it would ruin the worldview of the Pokémon universe. Sugimori basically says that Pokémon should be earned and not bought:
Sugimori said that "when it comes to business, the one thing I've always said ‘no!' to is ‘the act of buying Pokémon with money.'"
"That is something that has been said since the days [Satoshi] Tajiri was completely involved in everything," he added.
"The reason being, is because it's one of things that could ‘ruin the worldview' of Pokémon," Sugimori he continued. "I believe the reason we don't simply commercialize [Pokémon], is that it's a way of protecting the brand, and for this purpose, we have the specialty company called The Pokémon Company."
"Therefore, suppose we sell a Pokémon for 100 yen, then we must prepare something that is worthy of that 100 yen, along with a reasonable consent for doing so."
Sugimori went on to defend the practice of giving away free and rare Pokémon as promotional items at events such as movie screenings, saying that the idea seemed fun at the time, and often served to give players the chance to play with the rare Pokémon that they just watched in a movie.
The latest Pokémon games, Pokémon X and Y, has sold over 4 million copies worldwide in their opening weekend.