The series continues because stupid little goons like myself still watch it >.<
Thing is, it has this... set formula. Ash leaves on his journey, gets his hands on one or two of the regions starter somehow, gets himself a "pigeon" Pokemon, goes on his futile journey, somehow beats all the gym leaders and gets into the league, loses in the league because he never learns. Rinse and repeat. I can sit through an episode of Pokemon, know what's going to happen, and somehow still enjoy it <.<. I think its people like me, who never get bored, are keeping the show alive (must be loads of peeps like this in Japan).
Strangely though, i don't see every episode as a filler like most people do. Throughout the years I've stuck with the logic that if Ash registers a Pokemon in the dex, one of the casts Pokemon evolves or learns a new move, then it counts.
I also read somewhere sometime ago that the producer for the show once said: "The day Ash becomes a Pokemon master is the day the show ends". It was at that point i realized... Pokemon isn't going anywhere now is it ;)