The creators of anime obviously are wary to launch a long TV-series, and so they launch a projects in the 12-series format. In case of DW the material here is quite a lot and it’s good. So why not just make 24 episodes, instead leaving the viewer with a feeling of walking on a street that suddenly rests on a dead end? Even Ao no Exorcist, which for me was the main disappointment of the season, has 24 series. Although, it is likely, that such not very good examples from colleagues are making the influence on the decisions of producers to begin with “single season start” option.