Steven stumbled upon the GS ball and a plot hole opened in the floor of the cave and he was sucked in, never to be seen again
Maybe the writers heard about how unpopular his anime incarnation was (probably due to being intensly stupid), and decided not to take any chances.
if the new champion in D/P doesn't have a HUGE recurring role, pokemon sucks.
And I will telepathically glare at them for all eternity because of that. Oh yes.Same reason they never showed the Johto elite 4 in the anime: they just dang didn't feel like it