I get it too, personally, I'm not either. I find her much hotter in something like this but I appreciate that the onesie help highlight her figure.
I think it's also more than just her body, it's the all the parts together. While her body screams hot...her facial expression and her pose is very cutesy, I'm betting if it was just a photo alone, it might not go viral as the video would since it captures her movement as well.
See I was thinking you would show this as your example