Why some pokemon doesn't evolve? Because they're marketable and merchandise plays a big part on pokemon merchandise. Now it's worse because Ash is the only star and because the writers are more sensible to keep as much marketable pokemon as possible. And because 90% of marketable pokemon are unevolved pokemon (or pokemon born to be in sales, see you Emolga) some pokemon remain unevolved.
Because they're too sensible, they are keeping the plushies' team intact with the only exception of Pignite and because the league was near (if the league wasn't near, I completely saw more time of Ash's Tepig).
and @dman_dustin, you're wrong that the decision of making some pokemon unevovled are BS. That was the inside reason of this, but they're doing this to explain fans like you that they aren't evolving some pokemon because they'll find very important to promotion.
-Pikachu is the FACE of the franchise. Enter something with pokemon and possibly the first thing that you'll see is a Pikachu!!!! Sorry, but no. Also, Raichu isn't going to be a Pokemon mascot (I want that Raichu returns to being in sets to troll people thinking that Ash's Piakchu are evolving)
-Bulbasaur was a popular pokemon that now it's getting some merchandise via PC and what not. That means popular.
-Piplup. This guy is so marketbale. He is possibly selling more than 2 times the amount o merchandise than Axew "in the second part of 2012!!!). Yes, in Best Wishes series. That's a lot for a promotion for a pokemon that eblongs to a older main character.
I think that when a characters leaves they planned to be permanently and not thinks of a character are returning. I only see two main reasons to return a character in the pokemon series:
-That the character ithat replaced him was unpopular to begin and the decision of leaving was bad writing at the finest (Brock's situation in OI.)
-THat his main pokemon are selling more merchandise than the main pokemon of the new character in the series where only the latter are the main character: Here is the catch: Piplup is more popular than Axew now. (Note: Remember that BW1 was a special case in marketing, Axew was more popular here but for obvious reasons)
Marketing of Piplup is the sole reason that I see a older character returning to the show full time. I alkso think that Dawn was way more popular and Iris (I think that Iris wasn't very popular until the game counterpart was revealed to be Champion: the CHampion status has helped in Iris' popularity, trust me).
And I completely see for marketing reason that the main stars of the main cast of Gen VI are Pikachu, Piplup and Eevee.