There were some reviews back when that were critical of the 71's looks: grille, billboards, etc.
Has to been a more complicated grille to mold nicely for sure: so maybe bean-counters.
Same as for the 70/71 challenger grilles: a molded grille sure is cheaper then the 70 egg-crate one.
Early 70's chrysler was still coming up to speed utilizing molded plastics in their cars.