well because a model is suppose to be different from other women, to stand out that´s mainly why the taller the better, but i know girls that can´t model because they are very tall, for example i know a girl who cannot model anymore because she is 6´5 or 7 something like that so that´s too tall to be a female model, but i also know models who are 5´7 or a bit shorter and have an amazing face structure and they are hired as models. so it really depends on the girl and it´s propose, smaller girls aren´t used for runway but mostly for photo-shoots while taller girls are used in both hope that helped 🙂