Out of the Silent Planet was actually announced as a movie back when I was in college (about '97 or so). Nothing ever came of it.
I've liked the idea of seeing The Space Trilogy as movies, ever since reading them for the first time, but there are several potential roadblocks.
In the first place, Lewis wrote them as H.G. Wells-like travelogues. He's very interested in capturing the sense of the scene and the atmosphere, not so much with moving the story forward. That works well in a book, but it can be deadly for a film.
That doesn't mean it can't be done, or be done well. But it certainly does make a very difficult challenge for the filmmakers, so much so that this is likely why the film never got made.
Keep in mind that Ransom doesn't speak to another soul for about half the book, and when he does, it's an alien, and much of the story is just about Ransom learning to speak the language. Again, great for a book, not so much for a film.
Someone I knew once explained it to me this way: Books are what people think; plays are about what people say; films are about what people do.
A story can make a successful leap from one medium to the other if it has enough of the other appropriate elements in it. (I think the CoN, with their visuals and action, stand a very good chance of making this leap well.)
Of course, maybe that wasn't the problem at all. It may have been just that the filmmakers wanted to remove any Christian elements from the storyline, and Walden Media wouldn't have it. Who knows?
Re: LOTR movies --
The movies are handsomely filmed with truly ingenious special effects, and the fact that Peter Jackson was able to capture such an epic story with any success is impressive enough in itself. Howard Shore's score is fantastic; I play it all the time. And there are parts of the movie, particularly the first half of Fellowship of the Ring, that are just perfect.
However -- and this is a big however -- Jackson largely missed the themes and the emotion that have resonated so strongly with readers for 50 years. When asked by one interviewer what the films were about, Jackson shrugged and said, "I dunno. Environmentalism, maybe?" When further asked if he was interested in preserving the Christian themes that Tolkien put in the novels, Jackson replied, "Not a shred."
Of course, this is frustrating from a Christian's perspective. But let's take a perspective of just people who liked the books, Christian or not: how can a man spend 7 years of his life making essentially one long 12-hour film and have absolutely no idea what the story meant, even for him personally? Why even bother, then?
The acting was wooden, particularly from the elves. The noble Gimli was reduced to being the butt of numerous mean-spirited "short person" jokes. And I'm not even going to get into all of the character inconsistences and "moving around" of the plot, because I understand that the filmed version must be different from the literature version (for the reasons mentioned above), though I disagreed with nearly all of his changes.
Lastly, Jackson's idea of "a touching ending" seems to be one slow-motion hugging scene after another. How long that did go on in Return of the King, anyway? 25 minutes? A friend of mine reported that at the showing he saw, people kept getting up and leaving, thinking the movie was over. Finally, the audience began laughing at the supposed "emotional climax" of the movie!
I had been waiting for a sci-fi/fantasy film to finally win Best Picture, but ironically, when Return of the King swept the Oscars, I was disgusted! I can't think of a nominated sci-fi/fantasy film that was less deserving than Return.
(Nominated sci-fi/fantasy films from previous years included Star Wars, Raiders of the Lost Ark, E.T., Disney's Beauty and the Beast, etc. These were good stories, well acted, well filmed, with a good sense of theme and an emotional connection for the audience. These films didn't win any major awards, and in fact only won awards for music or special effects -- ironically, the only things that LoTR did really well.)
Whew. OK, I'm done ranting for now. ;)