One might look at the other books to understand the constant "falling away" of the Narnians. Quite a representation of the Bible that constantly showed the Jews falling away from God.
When the children arrive in LWW, many doubt. That may explain why so many turned to the White Witch. That Aslan walked among the Narnians helped reinforce their belief. While Christ walked the Earth, so many gathered around him. After his death and resurrection, it cannot be stated that everyone who followed him remained loyal to him.
In Caspian's day, just two hundred years after Aslan's saved Narnia, the Narnians had fallen away.
And so it goes on, like I stated in the first paragraph.
Perhaps so many of us feel that Christians have not fallen away. Yet, a scant one hundred years after the resurrection, Christians were killing Christians over "correct" doctrine. Witness Paul's letters to the different churches to correct their individual wanderings. And, we Christians have divided ourselves over doctrinal and personal issues to the point that, in my opinion, we have lost the fullness of the Light. Let me repeat that: the fullness of the Light.
IN the last battle prophesied in the End Times, we should not be surprised if many people we know and love turn away because the End Times does not pan out the way each of us individually believes will happen.
I feel Tirian is in a dilemma. He feels that Aslan, like a deus ex machina, will come and straighten everything out. He underestimates the strength of Narnians, that once shown the truth they will turn out en masse behind him and the banner of Aslan. The prophets Elijah and Elisha along with the great champions like Samson had to feel helpless at times as well, as the Jews would not align themselves to God even after He showed His power and majesty.
I cannot fault Tirian for his humaness: he is truly like the rest of us, trying to do good when some actions may just lead to doing something harmful to others. The positive part that comes through is that he never lost faith right up to and through the end.