To put it bluntly, the longer the focal length (and the greater the distance to the subject), the less interesting a portrait will be. But coming in close is too confrontational for most people, and takes away form the flattering image most customers of a commercial portrait photographer will desire.

Then again, I just happened to see Arnold Newman's "portrait" of Piet Mondriaan. Quite distant, but the composition really makes the image.

Just saying...