Why do eating disorders affect more girls?

Eating DO. Pursuit of thinness, self-image & self-esteem issues are more in females battles.
Not sure. You are absolutely correct that they are much more common in girls, and many explanations have been offered ranging from social pressures to effects of "female hormones" to personal psychology. The cluster of onset around the time puberty sets in doesn't help much because all of the above and others place disproportionate strains and stressors on girls that may lead to eating disorders.