Simply the passage of time is wearing away at its popularity...as with all things, pokemon has flourished as long as it's going to flourish and now it must take its place on the back-burner of popularity. It's still popular by many standards if you ask me, but it is no longer a craze. The aftermath of the craze, which peaked about 2001, is a widespread fanbase which will be devoted to the series, games, etc. until it all ends (which will likely not be soon...I still see young children who are very much into pokemon.) No one factor can be blamed for the decline in the popularity of pokemon. The anime is actually rebounding from a decline (new area, no more filler episodes) actually...and pokemon was actually never the "in" thing...people were always ashamed to bring pokemon stuff to school (it has always made us a target for persecution.) Things are basically as they've always been, except toned down somewhat. The toys and advertisements and promotions aren't in our face anymore, that's all...and of course some people have outgrown it, but the popularity isn't all that much diminished. I'd have to say there's nothing to worry about...pokemon still has a long future ahead of it, just one less widespread.
