Isn't it based on a comic book series? Perhaps they are giving the comic a bit of lead time so they don't have to make filler episodes while the "real story" is being written.
Though, zombie killing filler is pretty epic....
Based loosely on the comic book. As someone who has read them all, it could take 3 or 4 seasons to catch up if they followed the story. If you like the TV series, buy the trades. The story is different enough it won't spoil the TV series but is still great.
