Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Why wasnt this included in previous OS?

Presumably because it wasn't ready. They mentioned on the talk show podcast that there are some peculiarities in the way Siri needs to work on the Apple TV; people are more likely to say sentences with two languages in, say, French with an English film title.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.