That we hardly knew her before LoC. She isn't anymore obvious than the other Forsaken suspects. For example, Sammael seems even more likely than Graendal. Thus, she fails the intuitively obvious-test, if there are other candidates who seem for basically the same reasons more obvious that she is.

People just jumbed on Graendal being the killer, because other Forsaken can be ruled out in LoC, while Graendal seems convinced in LoC that Asmo died (of course she also might have learned this e.g. from the killer, the DO or Demandred).

Aside from that, there are the many other reasons I already pointed out why the "Graendal wanted to Compel servants"-scenario hardly makes sense.

And if Graendal had really done it, it most likely had been already revealed in her last PoV in TGS. Since Graendal is probably dead now, it seems quite unlikely that she will have another PoV from before her death or that it will be revealed by someone else (RJ stated the mystery will probably be revealed in the killer's PoV, which naturally would have the most impact).