The reason is... - Edit 1
Before modification by Etzel at 29/12/2009 12:05:55 PM
I still don't see the logic of saying that someone couldn't have murdered another person many months ago just because that person may have been recently killed. Being killed doesn't negate actions you took in the past, and even the most powerful stream of balefire wouldn't undo her actions from months ago.
That RJ said that he will most likely reveal who killed Asmo in the killer's PoV. Since Graendal will most likely not have any other PoV, it makes it less likely that she is the killer (it's also not really understandable, why it wasn't already revealed in her previous PoV then, since she is among the Forsaken with the most PoV).
But even without this the Graendal theory was basically simply based on the fact that we see her as one of the Forsaken in TFoH before Asmo's death, who wanted to lure Rand to Sammael in Illian (at least, if you take the clues from before LoC, as RJ said we should).
I just don't see anything beyond "Slayer slays things so he probably killed Asmodean! Oh, and he has powers!" to this theory. Sorry!
Slayer is a Top Assassin of the Shadow with special "channeler-like" TP abilities by the DO, who already hunted/killed other important traitors or enemies of the Shadow before LoC.