Well its unusual to be that thin and its chic for the designers to have these awkward, sometimes even ugly, unusually tall, breastless women model their clothing on a runway. For one, look at the type of people you are dealing with making these clothes. They want to be weird.
Secondly, they also like these women to be roughly blank slates for the clothing they are going to model. They call them "Hangers" in the industry because they arent there to show off their own bodies. They are there to show off the clothes.
Thirdly, the designers are able to fabricate a standard size for the runway shows and then all the women are able to wear them enterchangeably. They don't have to worry about getting a size 9 for Misha and then a size 3 for Tory. Get the idea?
When everyone is tall and lanky they can wear the clothing, almost loosely to allow for movement in the clothing to look good on the runway.
They do have a limit though, even the most posh of designer wouldnt take on a model that looks like they are going to die any moment of starvation. Even though more women are told to lose weight, they like to see some bone and have a high tolerance for it. They will every now and again tell a woman she is too sickly to model for them. They don't want too much negative attention brought to their line.
Back in the day looking beautiful and having symmetry to your face would get you work. The days of Nikki Taylor and Cindy Crawford are long gone. Now if you have unusually bizarre features, like a huge gap in your teeth, weird freckling on your face and a big forehead and super wideset eyes....you can look like a character from Avatar and get more publicity.
