Quite simply the news did not travel. Tirian was occupied in a distant part of Narnia. The farce carried out was initially quite localized. The other animals were fooled into obedience by their smarter leaders and convinced that they were serving Aslan.
Comparisons: The slow travel of letters between Britain and the USA allowed the fomenting of the Revolution.
Comparisons: The German people were in a state of economic and political disruption, relatively isolated from the rest of Europe by law and postWWI restrictions and payments, restive under an resurgent nationalism, and LISTENED to Hitler. Even when the rest of the world became aware of him, he was considered an aberration and unfit to listen to, so why would the Germans believe him? But they did because of the circumstances, didn't they?
Comparisons: The re-interpretation of Aslan began in a localized area under the confined communication conditons and the affected area was thoroughly entrenched in the falsehood before Tirian could do anything to counter the false teaching. (To be sure there were individuals who weren't convinced by the Ape and the Donkey-in-a-lion-skin. But they were squelched, weren't they.) This is not unlike th proliferation of sects under the relatively isolated conditions of settlement of the American frontier and the evolution of Mormonism and Jehovah's Witnesses in the recent past. Nor is unlike what the Episcopal Church and the Anglican Church in Canada are attempting to foist off on the Anglican Communion and Christian Church throughout the world.
Unfortunately, Roseymole, Mr. Lewis has demonstrated an altogether too common scenario quite likely to occur in any world like ours (that is, disobedient to its Creator).