The great thing about Kiva robots is that they're quite simple and robust mechanically. They're a battery, motors, wheels, and a lift mechanism. They're completely interchangeable, and having one fail isn't a big deal. Maintenance is replacing batteries, motors, wheels, and maybe the electronics module as necessary. The infrastructure they require is modest. The competing systems all seem to require more elaborate robots, special totes, and special shelf construction.
Amazon says that Kiva robots cut their order picking time from 90 minutes to 15. Other retailers, seeing that number, must be terrified. It's a technological improvement like that which makes same-day delivery from a large inventory possible.