IT was always expected to be a movie that did well, but nobody was quite ready for just how big a movie it has become during its theatrical run. Many explanations for the success have been given, but the director has his own ideas regarding why his movie has worked so well. Andy Muschietti thinks that what sets this film apart is that it is a rare horror movie where the audience actually connects with the characters. According to Muschietti...
I don't think [audiences] quite expected such an emotional connection with the story. That's what I take from, not only working on the movie, but the reactions. When I hear them, and I read them, people are very attached emotionally to the journey of these kids. Which is not really frequent for horror movies.
It's certainly true that building emotional connections to characters in horror movies is often a waste of time. In most cases, horror movie characters are little more than cannon fodder. Their purpose is to be brutally killed by some monster or psychopath. However, IT was not that sort of movie. As Andy Muschietti told i09, the story is really about the kids, who find themselves the target of a supernatural monster and must work to do something about it. As you follow the kids on the journey you become invested in that journey, and that goes a long way to making people feel like they've spent their time at the movies well.
There have been numerous theories about how IT managed to blow all of its expectations out of the water. Only a couple of weeks before the movie was released it was looking at making something around $100 million at the domestic box office during its entire run, yet the movie was able to do that in one weekend. The early buzz was very strong. Critics who saw the movie early were nearly universal in their praise. It should also be pointed out that early September was not a big period for major film releases. The only other wide release the weekend IT came out was the Reese Witherspoon comedy Home Again, and last weekend's American Assassin and mother! also failed to ignite audiences.
These were both absolutely factors in the film's success, but they can't be the major reason. Movie fans could have just stayed home if there was nothing else at the theater they wanted to see, but they didn't. They made a point to see IT. Oppositely, many critics liked mother! too, and that didn't help that movie much at all.
Ultimately, IT is a horror movie, but first and foremost it's a character story, which means you can enjoy the film even if you're not traditionally a fan of the genre. Said the director,
IT is a story that can be told without that fantastic element. It's a story about a group of kids who are lonely and oppressed and they learn to get powerful by getting together. So the fantastic elements are sort of on the backburner and that's why people connect [with it]. It's a human story.