There are two basic types of aircraft wifi, Air-to-ground (ATG) and satellite. ATG seems to be more common. It's probably cheaper. Obviously it works only over/near land, though, and even then it can be spotty wherever there are no towers nearby— which is quite a bit of the mountain west, actually. One source I read said that Southwest Airlines uses a satellite service for all of its flights. The aircraft I flew aboard today definitely had satellite wifi as I used it throughout our overwater trip.