The reason is very simple. I'm not choosing the stainless steel because of its rarity, all Kari watches are rare to begin with.
I just want a very basic, non precious case to be a body of a special Observatoire movement. I think the conrast will be nice.
Also, Kari told me that for SS case, he will be doing it himself and not outsource, that helps to convince me too 
Lovesss*CL