Season 5 (Spoilers... I think?)

Think about it. Production, voice recording, story boarding, SFX, Music, character development, ANIMATION. For the most part, a season is almost fully completed by the time it starts showing, that way a schedule can be made. 26 episodes times 22 minutes. each minute is about 1,440 frames of animation at 24FPS. that's 31,680 Frames for one episode, clocking around 823,680 frames of total animation work on the show for a season. Plus they can't re-use all there symbols. surely they will have new ones to make and what not. Sure there is still the second movie coming out, but as for the 5th season, a lot of work has to be put into it, as i'm sure licensing, and other legal and behind the scene stuff.

So, if you're asking why 2015, then that might help. A lot goes into something, and i would rather they take there time with it, then rush it.