These kind of questions come here over and over when we talk about adaptations (books to movies, games to movies, comics to games, etc.) and it's really simple to answer:
1) Because movies aren't the same as books—the same applies to comics, games, manga, anime, etc. etc.
Really, keep in mind that maybe the book could be a really incredible experience to be read. But as I said: to be read. Trying to copy and paste the narrative mechanisms to another kind of art is very tempting but it could be a disaster, too.
Talking about the subject in question, The Hunger Games, it's very easy to read because Katniss explains everything (one of the reasons, but let's stay only with this for now). That's because the reader simply watches Panem through Katniss' eyes, and lacks the need to construct explanations. In the case of a movie, the images explain themselves.
Could you imagine two hours of movie with Katniss saying things like "I'm hurt", "now I'm hungry", "now I'm falling in love with Peeta... or maybe not" and the sequence of images showing the exact same thing? Boring.
2) The reason is way more psychological: Collins and the director (Gary Ross) have two different ways to telling the same story, and really that's it. Two different POVs, two different narrative tools, two different minds, two different ways to show us the same thing....
Personally, I like both the book and the movie, each for its own reasons. Both the movie and the book show a really dramatic image of a very degenerate world, but both do this in its very unique way.
PS: sorry for my English, I'm just learning, I hope you understand what I wrote.