I guess that was a question of available personnel. Gortash would have needed to train high-level Bane cultists who would be missing elsewhere, or potentially unreliable Zhentarim mercenaries with ulterior motives, for a highly risky mission into astral space, piloting a vessel able to navigate the hazards, and fighting Githyanki in one of their mightiest fortresses, all while trying to keep this secret from his co-conspirators who did not seem to be as knowledgeable about the prism as Gortash. The training would probably have taken weeks and months and they may not have had the luxury of so much time. At least the Sharrans were on the trail of "the weapon" as well. Mind flayers, on the other hand, would have no doubts about their loyalties to the grand design, know their way around the plane and how to fight Githyanki.
