It's simpler than that. Covers are designed to sell books. Seven was the most popular character in Voyager. Therefore putting Seven on the cover sells more books. It's the same reason they put Worf on the cover of The Struggle Within even though he's not one of the central characters. And the same reason they put Spock on the cover of Cast No Shadow even though he only has a cameo role. The primary consideration isn't how well the cover reflects the story, but how effective it is at attracting the customer's interest.