Harold Tessmann III
I thought the question would be "why is it only sold in US" - it doesn't seems to be a lack of rights or interest that prohibits from being sold outside US.
CGE tweeted that a representative of Target, a retail store chain based in the USA, proposed the idea of this version, and so Target got the distribution rights. Target hasn't released the game to the American hobby game distributors (I did ask my FLGS), and as they only has stores in the US and Canada, I don't know if they'll go to the effort of making deals with distributors in other regions. That said, the game has no mention of Target on the box or in the rules, so it doesn't sound like they own the game itself, leaving open the possibility that CGE will distribute it more widely later on.