Yang Kai didn't bring too many people with him. He needs to guard the gate as well, all the time, because otherwise the folks outside could send more people in, the gate being located in a hostile place in the Free and Unfettered Spirit Temple's land. Unfortunately his forces back in the Holy Land aren't exactly huge, so how many powerhouses could he spare to take with him? He could have taken at least a bunch of third stage transcendents. Surely there are a good number of those among the humans and demons.