Although I have always respected the decision and individual criteria of the managers of the signature campaigns, I believe that only selecting participants who have a specific nationality does not make much sense. I don't know who you are referring to, but any user of any nationality should have the same chances of participating. There are good users in every country in the world with opinions as valid as any other.
Sometimes interaction in the local board may be a reason for giving priority to the participants there. For example, the Russian and Turkish languages had high interaction in bitcointalk. Sometimes campaign managers prefer diversity so that one member is added to each local council or specific council, and so on.
Therefore, when there are a greater number of open slots available, the chance of you joining a campaign will be higher based on normal criteria such as
karma, activity, popularity, trust, etc.