Pokemon series art director Ken Sugimori said in an interview with 4Gamer that DLC for Pokemon would "ruin the worldview of the Pokemon universe." In summary, he said that Pokemon should be caught and not bought.
"When it comes to business, the one thing I've always said ‘no!' to is ‘the act of buying Pokémon with money," Sugimori said. "That is something that has been said since the days [Satoshi] Tajiri was completely involved in everything.
"The reason being, is because it's one of things that could ‘ruin the worldview' of Pokémon," 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."
So what about when Pokemon are given away for free as promotional items?
Sugimori explained that this practice was fun at the time, and they thought it would be a fun way to give gamers a chance to play with rare Pokemon they had just seen in a movie.
In other words, what you get with a Pokemon game is what you get, and the developer has no plans to ever sell DLC with new Pokemon. There's something nice about a game being a complete package.