Because it obviously would have a more powerful impact. If Graendal was the killer, BS just had the chance to reveal it in her PoV in TGS, prologue. Why then, should he skip this opportunity to let it e.g. Moridin tell someone else that Graendal did it? Makes no sense.

And I actually bet that no one, who thought Graendal did it, even considered the possibilty before TGS that the revelation won't be in Graendal's PoV. The idea that a third person will reveal the killer is just an attempt to rescue a wrong theory. Graendaldunnit is as dead as Graendal, IMO.