It probably has little or nothing to do with the story in the book. In the first place, that would be typical for a Hollywood treatment of any book. Additionally, this particular book doesn't have enough story to fill out an entire 20-minute sitcom episode, let alone a feature film.
> Take the Lord of the Rings for example, I remember the language and style
> of the Fellowship in particular being awkward and simplistic
Tolkien may have used simple language, but he didn't spend a page and a half detailing the appearance of a particularly mundane shrub in the dullest words possible. Also, not all of his characters were strictly one-dimensional and remarkable primarily for their unexceptional ordinariness. LOTR had a detailed plot, as well.
In terms of movie, LOTR had exactly the opposite problem of Of Mice and Men: it was fundamentally impossible to cram the entire story into a series of three longer-than-average movies. Even if they'd gone with five movies (one per "book" instead of one per volume), they still would have had to leave out a lot of the action.