In the early days, cars were more sensitive to weather conditions so they had to be stored in dry garage spaces that looked like any other building.
"As cars grew up, as gasoline became an all-weather commodity and cars became comfortable cocoons with sturdy metal shells, garages no longer needed to be impervious to weather. (...) A basic tolerance for ugly parking structures, a particularly soul-killing type of architecture that would blight whole neighborhoods and rend the urban fabric of so many once-walkable downtowns, had entered the American system."
Philip Kennicott explains the exhibit's thesis in full, and also pays a visit to the parking conference. Donald Shoup, of course, makes an appearance.