Threads for the entire show compared to episode threads suck. They have 0 flow and all context is lost in a mess of discussions about different episodes.
Episode threads are specific to the episode and you'll know the context of all the posts without being (accidental) spoiled about later events. It also allows people who don't watch the show seasonal (or get several episodes behind) to be able to partake in a conversation without being left behind and spoiled about later episodes.
And if you want to remember something that happened in an episode, its far easier to just search "Anime episode -1" and scan through the posts than to hunt through an anime thread. - especially for those airing longer than 1-cour.