I think they still use the National Anthem in the states, at least here in Texas as a way to haul out any dignitary or newscaster that thinks he/she can sing. What an audience these people have. Who else can say they've sung to 40,000 people?
I heard an amazing rendition of the Star Spangled Banner by one such singer. He started in a key that was just too high and the song stretches the top range. About 3 lines into it, he smoothly switched keys. I'm not sure if he knew what he was doing or not. I doubt that most people even noticed and his high notes came out just fine.
As to a real answer, I don't know and until this moment it never occurred to me to wonder why. Hmm! Now you've got me thinking.
Emily