@pwingbrony It might be that they don't think of their human bodies as being beautiful. As they used to have glorious, gleaming scales and suddenly… don't, they might even think they're all kind of ugly now, if only by comparison. I think it'd be like if you were morphed into a hybrid of man and insect; no matter how many other bugs/bug-people tell you your exoskeleton is the sexiest thing they've ever seen, I'm not sure how many of us would take such sentiments to heart.
Of course, that's just a guess. Could just be that the attention they could get for being pretty wasn't enough for the trio and they wanted enslaved masses, or even specifically to be adored for their voices, for all we know. :s