I think Moffat would prefer being his own boss on Sherlock on BBC rather than watering down the product to get a full season. Also he has got Doctor Who to run which he has loved growing up. Why would he want to get involved in anything else?
I don't think CBS wanted Moffat to come to run an American Sherlock
. I think, like NBC did with Coupling
, they just wanted the concept and they always had an American producer in mind for a present day American Holmes. And they also wanted to prevent any legal hassles and nuisance suits down the road.