Mainly because it is possible that Demandred has staged an uprising among the male channelers setting up schools similar to the Black Tower and overthrowing the female channelers that kept them down. In fact, it is possible that the Cleansing has worked in his favor.

The only point that makes me think this is not probable, is that we have no information as to whether or not Graendal kept tabs on the channelAyyad, or just the monarchs and nobles among the Sharan. My feeling is that she would have kept tabs on both, but I do concede that there are more villages and people than one channeler could visit, and it is possible that Graendal was working one end of the game, while Demandred worked the other unbeknownst to her.