My guess is two reasons, one is that Adriana's body is uh-mazing, but in this hyper-critical world of HD / HQ screen shots she has "flaws," and I don't mean that in a snarky way, but unfortunately if someone sees a minor imperfection it can become the topic of discussion, a posting on a site that specializes in mocking that sort of thing, etc.
My second guess, and more importantly, is that Adriana has come into her own as a spokesmodel and having her as one of the narrators may be more important than seeing her vamp in front of the camera, kinda like Heidi in recent times with VS. This makes me want to give Adriana a great big cheer because not only has she conquered her biggest obstacle, she is actually flourishing with that aspect of her job.