On the zeitgeist side of the argument, I could mention that, since automobile styling has a significant fashion component, the late 1970s and early 1980s marked a shift away from somewhat rounded bodies such as Chrysler Corporation's "fuselage" styling from 1969, Buick's Riviera with its curious "boat-tail" greenhouse, and Ford's Torinos. That shift wasn't all that great, but nevertheless a visible change to more angular bodies with thin roofs and proportionally taller greenhouses and proportionally shorter lower bodies. At the time, the effect struck potential buyers as being clean and crisp as opposed to soft and sometimes fussy.
There was another factor: the reaction by the market and the federal government to the 1973 oil crisis that for several months featured gasoline shortages and higher prices. In response, automobile makers introduced smaller, more fuel-efficient cars.
Finally, there was the growth of sales of non-American cars in the American market. At the low end of the size-price continuum were Japanese cars. But the higher-price part of the market saw inroads by German brands, especially in the form of the Mercedes-Benz W116 series. For some time, Mercedes cars had featured flat roofs and proportionally tall greenhouses. But now Detroit styling was adopting these styling cues, perhaps in part because of the fact that Mercedes was challenging Cadillac as the leading prestige mass-market brand in America.
And it is here that the zeitgeist explanation loses some of its force, because American stylists began mimicking Mercedes grilles on the Mercedes-like bodies they were crafting.
Here is a Mercedes that American stylists would have been familiar with when they were working on redesigns for the late 1970s.
Caprice/Impala styling in the late 70s wasn't as Mercedes-like as the designs shown below. But it nevertheless was typical of the times.
The Fairmont is technically a six-window rather than a four-window sedan. Aside from that, it follows the Mercedes formula. Note that the grille is horizontally sectioned into thirds using larger chrome strips, like the grille on the Mercedes shown above.
The Volaré and Aspen are pretty much the same car, part of the long-term Chrysler practice that culminated in the termination of the Plymouth brand. Here again, the styling seems Mercedes-inspired. Moreover, the Volaré, like the Fairmont, has its grille sectioned horizontally Mercedes-style.