Why? I think that Peter Jackson's films manage to bring LOTR to life, which helped to visualise some of the characters as I read the books. If a character wasn't in the films, that's fine - I just used my imagination.
But don't get me started on the Hobbit films ... oh boy. Dwarves bathing in the Imladris fountain, dwarves in barrels fighting against orcs, and dwarves in the Lonely Mountain recreating a giant gold statue of a dwarf - which then melts and falls on top of Smaug. And he shakes it off.
The rest of the film made sense, though. It just felt like PJ tried to do too much - i.e. the "silly dwarf" scenes (e.g. barrel rolls, fountain-bath, gold statue) were designed for kids who'd go "Wow, dad! Cool!!!", and the serious/horror scenes (e.g. Gandalf in Mirkwood, Gandalf captured by the incarnation of Sauron, Gandalf before the gates of the Lonely Mountain) were for people who had read the Silmarillion.
In short, I think PJ couldn't figure out who his audience was - so the films did too much, which irritated the people who knew the book back to front. I hope that made sense - it feels like I just ranted a little (sorry!) *blush*