What Gabi said is right...

There's also the fact that they'll earn about double or triple the money by distributing the Pokémon through many games instead of just providing a device that allows trading with old games. Plus, considering 99% of the Pokémon fans are Pokédex obsessives, it's an excellent opportunity. Right now, you need at least 4 games to complete the Pokédex (3 if you have acess to Special Events.). If they had made such device, their earns would've been cutted dramatically, regardless of how much the device could've costed...

Plus, they apparently wanted to start anew. They wanted to redeem the biggest mistake of GSC, and obligue you to try different things instead of trading your Powerhouses right away...

Pokémon is a cash cow. Deal with it.