fav infinity <3 i cant stop staring at the big ribbon and that boooookkk~ omgawwwsd~ is that sort of shining armor bibles? i dunno but im pretty much screaming like a crazy fangirl right now Eeeeeee you have such a amazing skill for clothes design. ya knowww x3 thank you so much for this <3
LotR was a miracle in itself. The fact that LotR was such a success has more or less doomed any future LotR movies for a long, long time--at least until people forget about them to some extent. Look at the Hobbit. Money got involved, the pressure from LotR's popularity got involved, worrying about public reception got involved, and the movie turned out to be a fraction of what it could have been. Spent too much time trying to make money and cater to fandoms, not enough time maintaining its integrity as a film. In the defense of those involved, it was nigh impossible to avoid these problems, largely because LotR was and is so well loved. The Hobbit was a decent movie, an entertaining one, and graphically/musically an amazing one, but the story was sacrificed, the pacing was sacrificed, and the subtleties of character interactions to an extent were sacrificed.
Sorry... didn't mean to go on a rant there. Just... tl;dr: book-->movie transitions are rough.
The difference is that LotR is a single story with a coherent plot and just a couple of interwoven narratives, while the Silmarillion is more like the Bible, a collection of stories, myths, historical and geographical explanations, poems and so forth, put together both in-universe and in the real world from a whole lot of different, often contradictory, sources that span a lot of time.
Now, if they picked just one story to focus on - say, Beren and Luthien, or the story of Turin - it could work. But if you want to do the whole of Silmarillion, then movies are the wrong medium. You'd do better with a TV series, a bit like The Game of Thrones.