Board Thread:Speculation House/@comment-14138255-20141031002615/@comment-24637175-20141102042136

Eliato wrote: It would have been nice if one of the ancient Grimm had decided to take a chance and join in on the attack. that could have be an interesting way to finish the volume with a tough battle between the ancient Grimm and everyone at the plaza and be able to get any idea on how powerful they can become over time.

Except that goes against their thinking as Ancient Grimm.

They are waiting for a chance to eliminate ALL humans, this fight would have likely not killed many at all, and since all older grimm have learnt then it's unlikely any of them would actually risk the attack.