I would expect part of its popularity, aside from the sentiments, is the fact that both lyrics and melody are very easy to learn and remember, lots of repetition, resulting in many people who don't make any effort to learn songs at all being able to sing this one.

I'm amazed some people thought the 'answer' was the American flag. If the song were written by a Canadian, I can't imagine anyone identifying our flag as 'the answer' to anything.