By using an android tablet they leverage the existing OS/UI/Java ecosystem to write a (fairly simple) app. Recreating that from scratch would require a lot more development.
Sure, it saves development time & money. But if they anyway need some kind of custom interface... problem is, the lifetime of the tablet is way lower than a corresponding "dumb" system. I'd see the advantage of using a tablet if you have lots of controls (let's say you control the temperature for 6 rooms, you can see more info, etc.). By the way not everybody wants to have yet another touch screen in their home, but that's another story.
