Because you can't improve perfection.
Snivy should have received some more attention and maybe even evolved. I'm still hoping she does evolve before the Unova saga is over.
Legitimately Caught All 649 Pokémon in White 2: April 12, 2013X/Y/OR/AS Living Dex Completed: November 27, 20143DS FC: 0103-9971-7893
Hmm, probably for the same reasons that Oshawott hasn't changed; kids like her the way she is and the writers aren't going to mess with a winning formula if it means that kids lose interest in the anime as a result. It all goes back to the popularity and marketability thing, which many people dislike, but it's just a natural part of this franchise and others like it.
I like Snivy how she is right now. I also don't think it was a bad idea to give Ash many more Pokemon than usual. I guess people really don't want him to "catch them all"; I know he won't at this point, so giving him Pokemon anyway won't make people happy.
~I just wanna feel that rhythm~
Snivy was mature and strong from the start so she really has no where to go except evolution, but the chances of that are um....
Besides Kingler, Ash has no waterpokemon that evolved, why is that? Kingler is his only pokemon who is evolved in the time beeing with ash.. Palpitoad was already his second form when Ash catched it..
I hope Ash will take older pokemon from Oak and lets them al evolve.. Then he has many good and strong pokemon!