will someone please explain to me why sigur ros is going to be on conan o'brien?

Well, they're not from America.

So they probably are not aware that they're going to be on a show that attracts its audience with jokes about the bowel movements of a dog puppet.

At least all the middle-school boys staying up past their bedtime (I can't imagine anyone else watching that drivel) will get to experience the joy of Sigur Ros.