Because they want the viewer to focus on the clothes and not them.
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.<br />
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.<br />
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...<br />
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.
Maybe they're worried about how it makes their cheeks look...Or maybe they don't like their teeth.
They are paid for the job they do..they are not in any competition of a thing..they do what they knw what to do well and leave the rest