Black Widow is more popular with the general audience because the general audience doesn't know Carol exists.
As for the very small comicbook fandom, none of the very many Captain Marvels, Black Widow, or Storm are popular enough to sell a book, except for Carol Danvers.
That's like saying Iron Man is just an inventor who helps people but not a superheroShe's just a skilled CIA agent who helps people.
Black Widow is a hero but not a superhero.