In the US, cars went from curvy in the 30s & 40s to boxy in the 60s and then back to curvy in the 90s. The price of oil, design imported from Europe, and fuel economy regulations all played a factor in the changes.
Update: A strong rebuttal from Jalopnik:
This Vox Video About The Evolution Of Cars Is A Complete Mess. (via @nick__vance)