"Competition from other forms of delivery. […] Netflix alone accounts for 30% of all internet traffic in the evening. That represents millions of moviegoers. They're simply not in a theater."
Ebert didn't really tie it into the overall argument as well as I would have liked, but he did bring it up.
Also, many people have home theaters that provide a very nice movie-viewing experience. So whereas a family of four might go see a special, well-reviewed movie like "The Muppets" in the theater, they may wait to see "We Bought a Zoo" when it comes out on video.