Nvidia graphics cards is AMD's choice. They have had years to get their drivers up to scratch. Historically Nvidia tend to annoy companies they work with, so if AMD can double their driver performance in the next 6 months then they are still up with a chance
Intel isn't much of a surprise either. One motherboard can take i3, i5 and i7 making the test machines simpler more consistent.
There is absolutely nothing to stop you from making an all AMD machine for SteamOS. Nothing.
Personally I will be watching with interest what they recommend, and will probably use an AMD cpu and use it as an excuse to upgrade my 2GB GTX460 as it should be enough for a 1080p TV and gives me an excuse to buy something new for my PC.
If you want to use AMD graphics, then right now their performance is half what it is under Windows. Only AMD can fix that, but you can just buy a graphics card that is twice as fast as the Nvidia one you need