Coming next Christmas:

Pokemon Black and White 2.5:

15 new evolutions!
40 never before seen Pokemone!
New Town and Gym!
30 new Pokemon trainers to battle!
New Items! New TMs! New berries!!
AND INTRODUCING NEW LEGENDARY POKEMON:
CASHILES AND CASHIMOR!!

They probably reject it because most of the Pokemon games, at least the main series, are on handheld systems which don't handle DLC very well. The market is also not in their spinoff games or the lesser known titles, but in the main series. They also make enough money on producing the same product over and over again with slight variations that they don't really need to do much else to greet their truckloads of cash.