The previous teacher provided a great response. I've been teaching this novel for over six years now, and have maybe only shown clips of the movie once or twice in my career, and then only as a time-filler (and only after the test since it's quite different than the novel).
Since Francois Truffaut's movie was released in 1966, we do not receive the benefit of "modern" film-making capabilities; therefore, we do not get the mechanical hound, the elaborate wall-sized parlor television screens, etc. We do, however, understand the "emptiness" of the setting, as the previous teacher stated.
Some of the major themes of the novel, I believe, are still intact even in the movie version: censorship can result in an ignorant society, and an unhealthy dependence on technology can lead to apathy and a meaningless existence, to name just a couple.
And you just gotta love the jet packs:)
Rumor has it a new version of the film is set to be released in 2012 (see link to IMDb.com). I'll be first in line without my "seashell radio" iPod anywhere near my person.
I haven't seen the movie in awhile, but no one else has helped you yet so I'll give it a try.
First, producers, writers, and directors normally try to keep a film as similar to the original book as possible. That would be the norm. So the short answer to the second part of your question is: because they could.
Now, just to clarify that, novels and films are two totally different mediums, so most of a novel is not transferable to a film. Most of a novel is left out of any film because the content simply will not work in film.
Concerning the first part of your question, the main characters remain in the movie: Montag, Clarisse, Beatty, and Millie. (Clarisse and Millie are played by the same actress, by the way: Julie Christie.) Firemen still burn books, the woman burns herself to death after she refuses to leave her books, Montag still undergoes transformation, etc.
In other words, there are many similarities between the novel and the movie. Perhaps one stands out: the setting is depicted in the movie much as it is presented in the book. Everything is barren and colorless, lifeless. Millie even has her TV walls to obsess over. The setting is joyless and changeless in the movie, as it is in the novel.
One contrast also stands out between the movie and the novel, however: Clarisse is definitely not sixteen in the movie.