Trying to find an illustration textbook for college students is difficult. In the textbook I am using, the author has an unfortunate approach to the history of illustration: two pages of personal choices leading to six pages of "outsider art." In his selection, the primitive gets highest value, for its enduring appeal to expressive types.
As in histories (skewed to modernist) of other creative disciplines, being hip is a big deal—it is the currency in trade. The avant-garde (hipper) favors the aberrant, which is picked up by the clever and voracious mainstream (hip), which soon becomes academic (not hip.) That which is unconventional quickly becomes conventional.
This pattern was first demonstrated to me when I worked in marketing. Back in the mid 90s we were tracking California skateboard culture for its unconventional fashion. Soon that fashion dominated the racks of conventional discounters such as Kohl's and JCPenney.