In these lists "of all time" tends to mean "don't let the hyperbole worry you, we're really interested in the last few years", and "best singer" tends to mean "one famous enough to have a fan base in the fields 90% of our viewers follow" (which tends to mean opera singers, Baroque cantata specialists, and divas of Eastern music must feel they've been wasting all those years of practice)
Still, the list being what it is, I agree that Carrie certainly deserves to be higher.
One of the comments probably explains why she isn't: "Carrie Underwood has one of the most beautiful voices I have ever heard. She is just underrated because she sings Country music."