I've always thought that the reason the humans were so important to winning the war against the Shadows (as the Vorlons had said to Dukhat over a decade earlier) was that they could tip either way. Human culture, compared to the other races, is very mixed and very dynamic and there's plenty for both of the Old races to work with. As a result, the humans end up splitting.

Add to this that one major race (Minbari) is definitely mostly pro-Vorlon, and one of the Narn/Centauri will probably go Shadow with the other being conquered as a result, and the only major race left is the key to the whole war.