User blog comment:Zathronas/Speculative author: Volume 2 and episode 12, general thoughts/@comment-10380701-20141101042050/@comment-24218038-20141101043440

The reason they chose volumes instead of progressing by season is because they have too much exposition to do before putting the story in gear in later years.

It's the reason why the grimm isn't the main baddy yet and Cinder is the main plot point for season 1... and the 3 volumes it contains.