Because its fucking amazing.
I just got back from seeing it and it is everything the first movie (The Lion, The Witch, and The Wardrobe) should have been. This movie is a lot darker, and while there still may be a lack of blood in most of the battle scenes, there is definately a lot of implied gore and a couple slashes. When someone dies, you know they did.
There's a lack of Mr. Tumnus (sp?). This could be a plus or a minus for you. It means that there aren't any horny goats running around after little children.
I don't do spoilers so I really can't say much else. Its a darker fantasy adventure, with a slight religous message that you can easily ignore if you choose to. There's a bit of comedic relief that lightens it up whenever it threatens to get too dark, but there is also a realism and a bad guy that's not pure evil, but just standard I'll kill you all evil but still be human.
100 thumbs up, plus they did what should have happened LOTR at the battle where the ents and their forest were supposed to show up, but didn't. Ok, small spoiler, but you won't mind.
Go see it.