My opinion is biased since I'm Canadian. Canada should so be in the game, and the CN tower should be a world wonder in the game. I don't honestly see why anyone would want to keep any nation out. Oh well I guess that's what modding is for!
It's not a matter of keeping people out, it's a matter of including a reasonable number of playable Civs in the core game without having to do so much work because the management is going to push the game out the door before it's ready. Additionally, Firaxis tried to give everyone a unique set of leader traits, characteristics, etc., while people just adding more guys to the game can relax that self-imposed rule if they so choose.
And you're exactly right--if you don't like Firaxis' choices (and I certainly don't on a couple accounts), you can change it!