Let's take out characters that have their countries name in their title though. Guardian's doesn't. He's not the 'Canadian Guardian' he's not 'Captain Canada' he's not the 'Red & White Guardian' - his title is simply 'Guardian' because for the reasons FP said above; he wears the flag and represents the nation. There's nothing to say an American couldn't do that.

Guardian isn't an iconic character to the mass market.

And if you were to ask half the world's comic readers what nationality Puck was, I imagine there would be a significant amount that would probably say American.
Apart from the stereotypical "Eh" Puck doesn't scream Canada.
(And there'd probably be a signficant amount that would say 'Puck who?')

But again, all my opinion and maybe I'm the only one in the world who feels this way.