Because they want the viewer to focus on the clothes and not them.
The whole point of the spoiled girl scowl of the typical high fashion model is this: She is signaling that she is so good looking and so fashionably and expensively dressed that she doesn't have to please anybody. Many of the models look angry, or arrogant or spoiled or sad and a bit neurotic. She is saying, "I am spoiled, demanding and I have a lot of baggage. But, the way I look, you'll just have to put up with it."
If that's all you could do for a living, would you be smiling?
It's actually because, psychologically, a smile is a sign of submission in humans. Models who smile on the catwalk immediately look self-conscious and awkward.
We see a smile as a sign of friendliness and approachability because it tells us that this other person is not a threat. That's why we like it and find it pretty. However, if you want to look confident and authoritative: don't smile.
Catwalk models are generally trained to keep their faces blank. It not only doesn't distract from the clothes, but it also makes them appear more confident and therefore makes the clothes look more desirable. People don't buy high fashion because they think it will make them happy. They buy high fashion because they think it will give them status. There's a difference.
Starving as a way of life will drive all smiles away...
But seriously, I think the answers above are comprehensive and pin-point the issue.
I think they want to draw the attention to what they are wearing, not to their faces. The whole point is that they are selling a style of clothing.
I think it's because they want to apear emotionless or even sad. When I see someone that looks sad Im atracted to them in the sence that I want to know why and to make them not sad, It draws your attention to them then they use that attention to get you to look at the clothes their waring. I allways look at the face first and the body second.
Maybe they're worried about how it makes their cheeks look...Or maybe they don't like their teeth.