They do have a CC for UK residents. I'm not sure if they have one for others.

I would suspect though that going through the process of partnering with a CC Bank for a specific country and validating all the legal stuff would be somewhat time consuming and cost a little bit of money. (I'm not in the Banking industry and don't know this for sure though.) Given that assumption, I would think that Marriott's willingness to do all of that would be proportional to the number of persons within that country that stay with Marriott hotels and would use that CC. And given that in general their non-US presence may not be as strong as their US presence, they may just think it is not cost justified.

But as stated they now have a CC for UK residence, so as they expand their presence in other countries maybe they will add more CC's for non-US residences.