I think the biggest reason is publicity and release date personally. LWW was all over the place when it was released whereas PC and VOTDT didn't get the same publicity coverage. Also they should have released PC in the winter instead of the summer.

They made all of the Harry Potter and LOTR films so why should Narnia be any different.

The Harry Potter and LOTR films all made a ton of money, whereas the last two Narnia films lost money.

The Harry Potter and LOTR films were never as overtly Christian as the Narnia books, nor did they worry overmuch about making the films suitable for children. Could it be, that, apart from LWW and its Christmas & Easter references, the other books deal with less popular aspects of Christianity, that, if not handled very carefully, will expose an adaptation to 'childishness' and 'cheesiness'?

There is another thing. C.S.Lewis, himself, and Tolkien had a difference of opinion about fairy tales. C.S.Lewis approved of Andrew Lang's collecting the folk tales he put into his Red, Blue and other fairy books and making them available to children. Tolkien didn't write LOTR with children in mind, nor did he think that folk tales should be 'dumbed down' for children. And JK Rowling, who wrote the Harry Potter books, has criticised C.S.Lewis, among other British children's authors of the time, for passing over the issues of adolescence she portrayed in POA, GOF and others of her increasingly darker books.

If the Narnia films were freed of the PG constraints we would have a very different sort of VDT, I suspect and certainly no problem in doing SC.

That is what I got out of that recent interview of Michael Flaherty's mentioned in the latest NarniaWeb news item. It seems that SC isn't so expendable after all. There is some pressure to do this movie. Though if SC is done, I can't be too sure that the other Narnia movies left would be done at all.