User blog comment:Xhatefate/DxD Updates~/@comment-99.227.35.241-20140110020357/@comment-1359322-20140111103333

That's cause Volume 3 & 4 are relatively easier to animate than Volume 5 & 6. They need more time to animate it properly plus the cost that they will used on it is still unknown but presumely higher than Season 1 & 2. That's why they are animating other anime, to collect budget to animate season 3.