Theory as to why: Carnage is considered to be far too violent (google some of his comics, we're talking HHN style gore here.) Venom, while far more iconic, probably wouldn't have appeared very well in 3D - he's mostly black, appearing in a city-at-night setting. Scream isn't nearly as violent as Carnage and has lots of bright colors (not to mention the hair) to cement her appearance. It doesn't hurt that she's considered a female (and therefore the only female in the roster.)