I've got a pet theory about this. It's got to do with the Paranormal Activity movies, which have been coming out every October for the past four years. The most recent one (the fourth) only did about half as business as the one that came before it. The exact same thing happened with the fourth movie in the Saw series, which declined in box-office grosses with every movie starting with the fourth one. So, assuming the same thing happens with the Paranormal Activity movies, when the fifth one comes out this October, it's likely to not do even as well as the fourth one did.
That means that there now isn't anywhere near as much competition for the dollars of horror-loving moviegoers in October 2013 as it looked like there was when the remake of Carrie went into production. So my theory is that Sony took a look at the numbers for Paranormal Activity 4, realized that the box-office in October of 2013 was wide-open, and decided to fill in the gap with Carrie.
Either that, or the movie is a turd and they wanted some time to try to fix it somehow. But since there has been absolutely no buzz about that being the case (like there typically is when a movie gets delayed for that reason), my guess is that it's because Sony feels very good about the movie, and thinks it can rake in beaucoup bucks come October.