Maybe

- Downsizing and weight reduction including crappy materials

- Aerodynamics

- Crumple zones and pedestrian safety

- Lack of inspiration for designers

- Programmed obsolescence mindset by the manufacturers includes making the car look old and ugly in a short time so they buy another one?

- Cost reduction. Maybe the average Joe car half century ago was not so average? I mean fewer people could actually afford cars. Or maybe not as even popular cars like VW beetles, Citroen 2 CVs, Minis and even Ford Ts had some appeal.

Of course all of the above matter but which ones carry more weight in your opinion?