On iTunes I see Chill Radio, Spa Radio, Down Tempo Radio and Electronic Radio which is pretty slow. I don't have Siri but maybe you should ask for one of those.
The point is Siri should have just a tiny bit of fuzziness so I don't have to say the exact right words. If I say "play classic rock" she does. If I say "play 80s music" she does. Not playing chill music when I ask breaks that expectation.
