I think that, technically, Tom was out because his participation in the massacre would have been a violation of Vito's oath. But such participation would have been a moot point given that a massacre was planned.
Realistically, Tom was out because, as I've posted several times before, Michael blamed Tom for Sonny's death and, in general, Tom's failure to protect the family.
"Generosity. That was my first mistake." "Experience must be our only guide; reason may mislead us." "Instagram is Twitter for people who can't read."
