It was a business move. See, what separated Pokemon from a lot of other games when it was released was that Pokemon was big on interaction between players. It had single-player that was fine on its own, but to get everything out of the game, you had to play it with your friends. What this did was foster community, a social experience around the product, and people with the game had reason to encourage their friends to buy the game as well. This is in contrast to most other fighting games or racing games or party games, where really only one person needs to own the game for all multi-player benefits to be available. This is why, as much as people want it, we will never get multiple save files in a Pokemon game.
According to Satoshi Tajiri, the idea concept of Pokemon was to trade data using two Gameboys, and a link cable. So, yeah Pokemon exist because of the idea of trading. No one will ever trade if all the Pokemon were available in the same game.
When did "casual player" become an insult? -Taizuku The real question is "When did hardcore gamer become a compliment/aspiration?" -JMBoote