That's an excellent question.
In community studies, it's about one in five, for men; in the clinic, it's about one in 30. The answer is that we don't really know. One of the theories is that its genetic, related to something on the X chromosome. Women have two; men have one. Other theories relate to patterns of brain development. There are clear abnormalities in neural circuit function in anorexia nervosa that probably pre-exists the illness. Some of those may be more common in women than in men.
It's really unknown at the present time. Certainly one of the important triggering factors, which is dieting, is much more prevalent in women than in men, although there is the computer geek thin for men as well, and many men who start weightlifting end up pursuing weight loss rather than getting bigger.
The actual answer is that we don't know.