They don't make "The Magician's Nephew" into a movie because Peter Jackson broke into the prop room and stole all the rings....
Actually, when looking at Narnia stories, they don't have the predictable sameness of Brian Jacques' Redwall (I'm a fan and even I admit it...as Yogi Berra once said, "It's deja-vu all over again!")
Some of the stories won't fit a certain profile that the makers of LWW have in mind. Even when punched and stretched to the point of shame.
The Magician's Nephew is in some ways a more psychological horror. You have to get in the characters' heads more, and that is difficult to write into a screenplay that would be superficially a lot like LWW.
On top of that you have a lot of extreme transformations that would be very expensive AND difficult to do in a way that conveys the right emotional touch. Such as the gold and silver tree from the coins. Some people may find things like this unintentionally funny.
And geez, create an entire universe from NOTHING while the audience watches. Biblical when read. Star Wars when visualized.
There is no single moment in the entire book that could not be done using existing technologies. It's a trick of doing them in a way that conveys the right emotional mood when you don't have an omniscient narrator saying, "She's calm on the outside, but inside she's all jangled like an overwound clock in a runaway freight train."
And don't look for that line...I made it up.