Because its simple. It wasnt a hard read so alot of people read it. I liked it because I like to place myself in their shoes and constantly ask "is that what i would do?"
When I say it's poorly written I don't mean that because they didn't use big obscure words, or have long philosophical soliloquies. I mean it's just poorly written, halfway through the book and I only knew 2 things about the main heroine, that she is a hunter, and she loves her little sister... and the book is written from her point of view, and I know virtually nil about any other character.
There are
PLENTY of books that are easy and simple to read for even 3rd or 4th grade level reading that are SO much better written. Take "The Chronicles of Narnia" for instance, super easy to read, yet still incredible books on so many levels. Or how about Brian Jacques "Redwall" series, or one more modern series I read some of just a few weeks ago, called "The Rangers Apprentice" by John Flanagan. They're are about a 4th grade reading level, and short enough I could finish one in >2 hours, and not really deep or philosophical at all, they are just fun entertaining stories that are much, much better then what I read of "The Hunger Games".
For crying out loud, even as much as I HATED "Eragon" for being a complete rip-off-mash-up of Star Wars, Tolkein and Ann McCaffry.. at least it was written in a coherent manner.