Back when I was an undergrad in college, I came across an absolutely incredible fantasy novel called Bridge of Birds by Barry Hughart.
Winner of the World Fantasy Award for Best Novel, it's set in ancient China and is actually one of the very few books that I have ever read twice. According to rumor, when Hollywood came calling to option the film rights, one of the producers purportedly made a comment along the lines of, "Does it have to have all these Chinese people in it?" No joke - for a novel set in Ancient China! (Although the producer did allegedly add, "Maybe we could set it in America.")
True or not, the statements attributed to the producer reflect a common attitude in Hollywood regarding films - namely, that movies generally stick to a particular formula. For instance, think of how many cop movies you've seen where, early on, the protagonist gets called on the carpet by his superior, declared a loose cannon, and so on? That's the formula at work; in fact, the formula is virtually the same for all movies. (It's also that same formula that makes filmmakers want to take a movie set in another country and transplant it to America.) In my mind, that's not really necessary when it comes to film adaptations of books - especially those that are huge bestsellers.
First of all, popular books already have a built-in audience. From my perspective, that's an incredible advantage for a film to have because some of the marketing/promotion has already been done for you (and will hopefully continue to work in your favor as fans get excited about the movie's release).
Next, if it ain't broke don't fix it. There's a reason why people fell in love with the book, so it seems counterintuitive to make material changes just to make it fit the Hollywood mold - like proposing that a film about China be set in America (I know, I know: I just keep coming back to that.)
To be fair, though, I understand the logic and mentality behind sticking to a formula. If a book sells a million copies at, say, $8 each, then it's quite likely to be considered a phenomenal success. However, if a film based on that book only attracts the same one million fans (who pay an average of $8 per movie ticket) then the film will probably be considered a monumental flop. Thus, I understand that some tweaking may be necessary, if only to broaden the audience base. (Moreover, you often can't transform novels containing hundreds of pages into a 2-hour movie without leaving something on the cutting-room floor.)
That said, I can't help feeling that something has gone wildly awry when immensely popular books have very poor outings as films. For instance, the film Beautiful Creatures (based on the novel of the same name) could only scrape together a worldwide gross of about $60 million - roughly the same as its production costs. The film version of The Host by Twilight author Stephenie Meyers only pulled in $48 million globally (which is probably better than Creature's haul, relatively speaking, when you consider that Host had a $40 million budget). Compared to those, the $90M worldwide gross by the film version of The Mortal Instruments: City of Bones looks like a solid home run - but that's before you consider its $60 million price tag. Again, those are not numbers to make studio fat cats greenlight a sequel.
Part of the problem, of course (in addition to chopping the novel down so that it can be made into a 120-minute film), is that much is inevitably changed from the book to the movie. Important scenes are excised, major characters become almost non-existent on the screen, and so on. In additon - and this is probably as big a reason for a flop as anything else - the movies are often marketed poorly.
By way of example, I read Orson Scott Card's captivating novel Ender's Game a long time ago, and then tore through the next 2 or 3 books in the series. (I know I stopped after Children of the Mind, but can't remember whether that's Book 3 or 4.) I was happy when I heard that a film version was in the works, and while I wanted it to do well financially the movie flopped at the box office, bringing in only $112 million against a production cost of $110 million. Personally, I blame the marketing campaign; despite my love for the book, I found the trailers for the movie to be ho-hum, to put it mildly. Although I wanted to see the film (and still do, which should tell you something), the trailers didn't really get me excited about the movie and make me want to plant my butt in the theater on opening weekend, the way I would with something like a James Bond movie. (Speaking of Bond, there's a formulaic series if ever there was one, but it works: suave leading man, hot chicks, cool gadgets, catchy theme songs and aptly named characters - 50 years after Goldfinger, I dare anybody to come up with a more memorable moniker for a female lead than Pussy Galore.)
In retrospect, I think Hollywood certainly has the right idea in terms of pursuing bestsellers with the intent to turn them into movies. I think the problem lies in the execution (eg, knowing what is and isn't essential for the story) and marketing. If you can get those two right, you'll end up with a Gone with the Wind. Screw them up, and you'll end up with something that's simply blown away like tumbleweeds and quickly forgotten about.