I wonder why? It's much more expensive, less powerful, has far worse specs and looks worse designed.
Mostly because Wacom's first party support. Plus there's been chatter on the Wacom forums that there will be an app/function that will allow the Companion to plug into a Mac/PC and work as a standard Cintiq.
Plus price wise, the entry Android model is only $400 more than a Cintiq 13HD.
The idea of being able to take a Cintiq with me to a meeting for presentation, then being able to use it as a truly mobile Cintiq when I'm on my laptop (because even the 13" Cintiq is not portable at all), would be worth the $400 upsell.