If it would have been a poem instead of a story, then so much detailing of the lesson would not have been possible and the length of the poem would have been very long. Deciding the theme of the poem would have been difficult because the reason of imprisonment was not very clear.
Step-by-step explanation:
If instead of a story, the author would have written the imprisonment of Moctezuma II as a poem, it would not had been a good poem because the exact reason of the imprisonment of the emperor was not very clear so it would have been a hurdle for deciding the theme of the poem.
Moreover, the detailing that are possible in the story does not become possible in the poem some times because the words and the rhyming scheme makes it a little difficult to inculcate everything in the poem which is possible in the story.