I agree with the commenters mentioning Arthur's lack of significance in the later seasons (Snowth & raidpirate, I believe).
Remember, Fernfern and the Mystery of Moose Mountain/Thanks a Lot, Binky? When I first saw those episodes, I realized that Arthur appeared in neither the intros nor in both episodes themselves. His only appearance was pretty much in the theme song. So if this was the first episode that a new viewer saw (who we can assume has never heard of the books?), they would probably wonder why the show was even called Arthur.
Anyway, I don't think the series was ever meant to focus only on Arthur; a show that did that probably wouldn't go very far. But if he is to be the title character, he should at least be given more emphasis than the others, while in the more current seasons, he's been pretty much just an extra or a sidekick to the main character of an episode. At least, that's my opinion.
I will agree that I've noticed a lot of episodes focusing on D.W. and the preschool class. I mean, we've had 4-5 episodes focusing on D.W. and the preschoolers in the first half of season 16 alone.