He doesn't use pictures because neither his target audience nor the subject matter require pictures.

His target audience is composed of people who are interested in mathematics teaching (parents, teachers and such). He expects (correctly in my opinion) that these have handled and can handle long texts.

The subject matter is an opinion piece without much underlying data, so graphical representation is not required.

The writing style must be adapted to circumstances. Increases in picture density do not monotonically increase text appeal.