Why Bacchus and silenus were used in the Narnia stories?

CS Lewis is drawing on a Classical tendency of early Christian Fathers (possibly Tertullian, Origen, etc.) who attempted to show how Christianity is not just revealed through the Jewish Old Testament scriptures, but in fact the figure of Christ is revealed in pagan literature as well. This was an apologetic attempt to show the revelation of Christ throughout all known knowledge of the time.

This is much like Paul of Tarsus in the Book of Acts preaching the gospel on Mars Hill in Greece using ideas and verse in their religion to communicate about Christ.

In regards to the use of Bacchus and Silenus, the Roman and Greek gods of wine respectively, I haven't read on what the Church Fathers would have said about them, but it seems that Lewis is drawing on their characters in order to point to the spirit of celebration, which is primarily what they are about. And in truth, what is more worthy of celebration but the advent or in-breaking of "goodness" upon a land and the expulsion of "evil." I believe Lewis sees the spirits of celebration found in paganism to be evidence of a deeper desire in the human spirit to celebrate goodness and justice.

For the Christian, that is Christ. The only truly good one.

It is fairly well accepted in literature that The Chronicles of Narnia are a Christian allegory, but the beautiful thing of what Lewis has done is appealed to our basic common desire towards goodness, justice, mercy, etc and drawn the parallels to Christianity.

This is also a possibility somebody else posted: Of what I understand of Narnia it was a story of children encountering talking animals and nature sprites and such creatures out of mythology and lore, the Narnia world at a whole seems composed of such lore and talking animals, it may be he liked to represent Narnia as the sort of world with faiths crossing into it from ancient religion and mixing with such themes in Christianity.