Kil'Jaeden somehow knew that the legion has a little chance to stand against 3 races that each has their own special powers, with that amount of demons and an Eredar (Archimonde) which is not even a primary leader (Just a field commander), a lich, dreadlord and a pit Lord as leaders, so he stood behind, not existing in the invasion.
But I think this time, he showed up because Sargeras himself is the leader.