Dash device is an exemplar of a service in the same way that a Kindle device is. Amazon retains the option to later go the other way too, but while the constraint is still on the supply side (limited markets) this device serves to fix the service as a thing and generate publicity.
Also, the platform is controlled by Amazon. Free from interference by Apple, Google, OEM or carrier. Wifi and internet aside, all support issues will be standard and coming back to the same place.
For a mass-market voice UI device with no screen, at this stage in the game, voice recognition has to be best in class. I doubt Amazon could have managed this through a plethora of mobile devices. And they would not want their commercial voice data to be processed on Apple's or Google's servers via OS services.
I expect a tear-down will show that some of the parts used are, for now, relatively expensive, but at volume, this device can become a give-away / throw-away.