Yea you're right Mark, there is at least 3 species of Elk including the Tule elk. I don't know why its not included in the Superslam. But there are several morespecies of whitetail deer, Bison, and even sheep.
I see that B&C even list Jaguar as a N American trophy as well as 2 species of walrus. I'm only guessing but I believe B&C drew the line somewhere, probably ruling that tule Elk, Jaguar etc are not included because of their scarcity and difficulty in being able to hunt them legally.