The long and short of it is the film failed for three reasons, none of them having anything to do with fans who read the books.
1. It isn't good. It isn't terrible. It isn't bad. It is simply isn't good.
2. The Studio didn't promote it at all. Thus, even the percentage of people who might have enjoyed it didn't turn up.
3. Aside from not being good, it brought nothing new to the table. This was Hollywood schlock we have seen a 100 times.
I'm going to be brutally honest here, tell the truth and shame the devil. The people who are most forgiving and seem to like the film are the very people everyone was certain would hate it. People who read the books by in large are forgiving that it bears no resemblance to them. Why? Well we had a year to kind of go through the stages of grief. We wanted to like it, so we kind of talked ourselves into it.

We aren't the problem though. The vast majority of popcorn devouring, soda gargling movie-goers are less well-read than we. They haven't read the series. Some might not even know there were books. They are judging the film purely on how it compares to others and whether it delivers the goods. It didn't. The audiences didn't warm up to it, or at the very least didn't find it interesting enough to drop their shiny coins in the bucket. Audiences, well-read or not, are much more sophisticated than anyone wants to give them credit for these days. They have seen a lot of movies. If you want to wow them, get them into a theater, and blow their socks off you can't just recycle the same damn story you have sold them a thousand times just by changing the names of the protagonist and generic, evil for evil's sake villain. Give them something they haven't seen, don't dumb it down, but sharpen it up and you put butts in seats.
At the very least, you have to show them something they have seen before but in a new light. Let's get down and dirty show we? How about I do a point by point breakdown comparing this film to the Last Action Hero starring big Arnold. It too is about a boy who travels to another world and brings a larger than life hero into what is supposedly the real world. That is also not a good film. My point is that it is almost the same story, albeit one played for laughs rather than melodrama. In fact, I'm betting if I put my mind to it, I could come up with TEN films which revolve around three main characters with one White Hat, one Black Hat, and one catalyst (male or female) and the stakes being the end of the world/universe. They would all have pretty much the same plot. Reluctant, jaded hero brought back to fight the big bad by the intervention of the younger, innocent or at least optimistic catalyst. I'll even go so far as to say the reluctant, jaded hero will be stoic and gruff with a heart of gold. The villain will be crazy and just appear to delight in being evil for no apparent reason. I could go on, but I won't. Most of you reading this know EXACTLY what I'm talking about and in the course of my description you are probably already on the way to filling out your own list of ten films.