Because slapping on a D-cup is the most unimaginative, childish way of making a Predator female. I've said it I don't know how many times before, but evidently it needs saying again - why the hell would a female Predator have breasts? Why would an alien species conform to human sexual dichotomy? Especially taking into account the Predator's reptilian features. Can you name any lizard species where the females have breasts? Of course not, because they don't exist. Humans are just about the only species on Earth, let alone in the wider universe, where the females have pronounced breasts.
Back to the pictures, I especially hate the hyper-sexualised look on display ion those images. The style of the art itself is great, but the subject matter is awful.