My hypothesis: because it doesn’t require a lifestyle shift.  Netflix is a subscription service.  Streaming video requires that you buy and connect television hardware, get a better internet connection, or upgrade your computer in some fashion.  Redbox works exactly like a video rental store, except cheaper.  You go to a physical location, get a physical disk, and return it to the same location.  There are no forms to fill out and no need to adjust your view of the world to incorporate the concept of mailing disks back and forth.  There’s no need to figure out why your internet is so slow, no problem if you’re watching the video in the car.  You don’t need to give Steve Jobs your credit card number, install iTunes, or even have a computer.

The most popular movies on Redbox are safe, family favorites because play-it-safe, traditional families are exactly the type to use Redbox.  They just want to watch a movie, without the hassle.