The baby boom brought a flood of new school construction. Then as the kid population declined many were turned into administration buildings, apartments, or just torn down to sell the land. Then the existing schools started to fall apart, requiring new construction levies that were hard to pass - "Why should I pay good money to edjumacate somebody else's snot-nosed kids when mine are all growed up!" On top of that, people were moving around a lot more often, so enrollments were hard to predict. Portables became the cheapest solution to make schools flexi-size.