Why did Ahasuerus never find a answer to his question?
This is a great question. The Sibyl by Pär Lagerkvist is an interesting mix of theology, philosophy, Christianity, Judaism and classical Greek and Roman religions. The work seeks to explore, in part, the meaning of life and suffering. It also seeks to explore man's relationship with God.
On one level, the protagonist, Ahasuerus is not given an answer, but if we step back, we might be able to argue that the lack of an answer is an answer. If Ahasuerus is looking for an answer to the meaning of life and the suffering of humanity, and in the end he is only left to struggle, the answer may be that life is a struggle. In connection to this, the idea may be that this struggle requires faith. When we look at the question from this perspective, there is merit, because a lot of religious people come to the same conclusion. Life is a struggle of faith. What is the alternative?
Finally, we should be prepared to accept the possibility that we might receive answers that we might not like too much. When this happens, it is still an answer.