Because film and comics are different mediums, with different strengths and weaknesses, and successfully transitioning from one to the other requires more than frame by frame reproduction.
Well yeah, I realised that. I'm not completely craxy. :)
However beloved any work in any medium is, I think it's a mistake to treat it as sacred text when adapting it for another. I want Snyder to make the best possible Watchmen movie, not the best possible Watchmen book on tape.
Trouble is, by not treating it as "sacred text" they tend to start screwing with it; it's one thing to take out bits but something else entirely when they start adding their own bits. Therefore, while I also want Snyder to make the best possible Watchmen movie, I want it to be Moore's Watchmen, not Snyder's Watchmen.
I'll cop to being one of the purists with regard to Eowyn's big dramatic revelation, but really, how much more screentime would five extra sentences by Miranda Otto have stolen away from all the CGI of vast armies massing and Legolas surfing down elephants' trunks?
I don't know about "Eowyn's big dramatic revelation", since I never got past the first movie but this is pretty much what I'm talking about. I love LotR and consider myself a purist but I'm not a craxy who learnt Elvish, or dresses up as Gandalf. I realise Jackson and Co. had to excise whole portions of the books to make it feasible to film it as a motion picture but as I've said numerous times; you don't cut stuff, and then add new stuff to the mix! More importantly, you don't completely alter the nature of important characters and have them do things they would never do in the book--for no apparent reason.
The most faithful book-to-movie adaptation I've ever seen is Battlefield Earth. Watching the film was exactly like reading the book.
I'm conflicted about Battlefield Earth. I depise Hubbard and what he created with dianetics, and I know the world would be a slightly better place if he'd stuck to being a successful author and nothing more but I do love this book. It's pure escapist space-opera, just like E.E. 'Doc' Smith's Lensman. I can't comment on the movie, however, because 10 minutes was long enough to make me realise it bore no resemblance to the book.