Briefly, the avantgarde promoting the blending of fantasy, SF and horror is exhausted. This is due primarily to the inherent poverty of their project. These (which are not even all the same kind
of thing) don't reinforce each other, they tend to cancel each other out. What's left is a sorry remnant.
There is also the exhaustion of their idols in the literary elite, who are worn out pacing within the narrow confines of their ideology. You know, human nature is eternal, the world makes no sense, people are bad, shit happens and you die, yadda yadda yadda.