I'm not a professional in the console world, but I have some theories on why a company might choose Android over a different OS or creating a new OS for their console.
- Name Brand recognition. Consumers with no knowledge of OSes will be confident that the device they're buying is running on something that's popular enough to be on their smartphone and millions of others.
- Developing an entirely new OS would add significant upfront and sustaining costs to the development of the console. Developing a new OS like Android is something very large companies with loads of cash do, like Google, Sony or Microsoft. Getting into the console market isn't easy or cheap. Using existing technology where available is an excellent way to reduce costs.
Android is a platform that many developers are familiar with already. No one is going to buy a console if there aren't any games for it. While mainstream consoles like XBox and Playstation have large companies developing games for them, the future of the micro console appears to be the indie developer. Indie developers run on shoestring budgets and tight timelines. They don't have time to learn entirely new OSes. The console developers need the developers as much as the developers need a platform for their games. Using Android gives both a way to make that relationship easier to start.
Android is linux already. Or a special version of it. So what are the alternatives? Microsoft or Apple would both cost far too much. Creating an entirely new OS is extremely expensive. So the alternative is to create another branch of Linux to use specifically for that console.
So what's the Android killer features that make it the OS of choice? Because it's there and it works. If it's not broken, don't fix it. If Android was placing significant restrictions on the developers to deploy the platform they want to deploy, then they'd use something else, so far, that doesn't seem to be the case.
(Valve would likely look at alternatives or creating their own to increase compatibility with their existing library of games).