A little joke by the New Zealand based game studio Ninja Kiwi.
- The game is called Bloon Tower Defense 6.
- The flags don't have game play impact it is just cosmetic / decoration.
- The money to buy the flags can be earned by playing. No real life money involved.
Yes it's odd. I don't see any pattern or rule. If they had added Ireland and South Africa then you could say that they intended to add the English speaking countries. And the other countries? I don't know. Maybe biggest playerbase?