I'd also be interested to know why they only choose four of the books,
The Lion, The Witch and The Wardrobe,
Prince Caspian,
The Voyage of the Dawn Treader and
The Silver Chair. Even if they were in fact limited to four, the question remains - why
those four?
The three which are left out are also the three which Lewis wrote last -
The Magician's Nephew,
The Horse and His Boy and
The Last Battle - though reading order is quite different. I can see why they didn't put on
The Last Battle; there is a great deal in that book which
could easily be interpreted as extremely bigoted in a whole variety of ways. It is also the most overtly religious; indeed, its arguable that it is a depiction of a clash between faith groups.
The Horse and His Boy could similarly be seen as racist for its depiction of a people who sound rather Arabic, perhaps even Islamic, though certainly not to the extent of
The Last Battle:
The Last Battle said:
...these men were not the fair-haired men of Narnia: they were dark, bearded men from Calormen, that great and cruel country that lies beyond Archenland across the desert..."Get on, son of sloth! Pull, you lazy pig!" cried the Calormenes, cracking their whips. The horse was already straining himself as hard as he could; his eyes were red and he was covered with foam.
"Work, lazy brute," shouted one of the Calormenes: and as he spoke he struck the horse savagely with his whip...
The Last Battle said:
...Then the dark men came round them in a thick crowd, smelling of garlic and onions, their white eyes flashing dreadfully in their brown faces...
Of course, this was written shortly before the BBC saw the arrival of the infamous (and at the time, very popular)
Black and White Minstrel Show, so perhaps Lewis thought little of it when he wrote those characters. In 1988, it would be inflammatory to say the least (true of 1988, but truer than ever today) to have a story in which the antagonists, frequently described as 'cruel', have brown skin, wear turbans, and worship a different god - the conclusions that could be drawn are obvious.
As to
The Magician's Nephew, it has a creation episode which again seems rather religious, even Biblical. England is generally speaking a secular country, so it may have been that the producers of the BBC series decided to go for books which were less obvious in their religious intonations:
The Magician's Nephew said:
...If you had seen and heard it, as Digory did, you would have felt quite certain that it was the stars themselves which were singing, and that it was the First Voice, the deep one, which had made them appear and made them sing.
"Glory be!" said the Cabby. "I'd ha' been a better man all my life if I'd known there were things like this."
The Voice on the earth was now louder and more triumphant; but the voices in the sky, after singing loudly with it for a time, began to get fainter. And now something else was happening..
Its hard to say exactly what Lewis 'meant', if anything, by his writings, particularly as they are children's books. However, it is a simpler matter to see what would and what wouldn't be appealing for a public broadcaster.