Because it was written first and they are designed to be read in the order they were written. 'The Magicians Nephew' was the 7th book that CS Lewis wrote, and it was written as a prequel to explain how Narnia came to be; why the wardrobe was a way into Narnia; and how the lampost came to be there; amongst other things. It is my favourite of the series, but it wouldn't be right to film it first.
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