>>12469
Part of what has changed design over the years is changing safety standards. The relevant regulatory bodies mandate that cars have certain safety features. The main reason pillars have gotten so much fatter is because in the mid/late 2000s the NHTSA or whoever it was, decided to require that a car's roof be able to support the weight of the car in the event of a rollover. Pedestrian and crash safety standards have dictated the general shape of a bumper, as they have for many years, but they have become more intrusive in the last 25 years. Airbag requirements have increased the mass of a car.
Part of the reason we got things like 50s American cars, and the M1 and the Countach is that regulations were fairly lenient to nonexistent, so the designers could do whatever they wanted - and they did. This changed when regulation increased in the mid 90s, which gave us cars that looked like the Supra and the EH Civic and the last generation Escort.
I'm not very good at explaining this part because I'm not totally clear on the idea myself:
Another part, in some cases anyway, is that the context in which cars are designed affects their design. This rule can be applied to the above - the reason cars look like the way they do is due to today's regulations. It's more than that though, because the regulations don't dictate exactly how cars have to look. I don't know what made pop-up headlights so popular in the 80s and 90s, but it was something that isn't present today - otherwise we'd still be making pop-up headlights.
A good example of context is American cars in the 50s. We had a lot of wide, open roads, and WWII just happened. We were prosperous and powerful, and car design reflected that. Cars then were gigantic and gaudy.