Highly likely a lot of it is simply older cars without safety features we consider basic these days (like side air bags) being phased out. Very few people driving cars from the 80s or earlier these days.
You can probably thank Ralph Nader for a lot of it, honestly.
Also encompasses the "cash for clunkers" program and period of low interest rates, both of which accelerated the retirement of older cars for newer (and generally, safer) cars
Yet in a lot of countries they don't anymore. Here mostly due to pedestrian and cyclist deaths being counted as motor vehicle deaths due to increasing car size and more km driven per capita.
A lot of improvements in vehicle safety were made between 1990ish and 2005ish. Around 2004-14 period the older cars that teenagers drive would have started being models with airbags and ABS. Neither of these have much impact on fatalities among "average drivers" but young driver's propensity to crash into things at very high speed and propensity to load more people than seat-belts into vehicles plays right into the use cases where these technologies will save the most lives. Cars from the mid '00s on up will also have much stiffer cabins because of a bunch of rollover and side impact rule changes that took effect around then.
Rate of fatal car accidents in US is about twice as that in Germany and Germans have autobahn with no speed limits drinking beer and then driving is part of their culture.
Car were safer and safer, ABS and stuff helped, Also the whole world started doing crash tests to help protect the drivers and passengers.
Note that the trend continued past 2014 in Europe (stabilizing now). Two differences:
- Less SUVs/light truck on the road in Europe (it's changing)
- crash tests in Europe test small cars against SUVs, SUVs against trucks... All different combinations. If your utility vehicle / SUV doesn't perform well on those crash tests, you can't sell it (hence no F-150).
You can probably thank Ralph Nader for a lot of it, honestly.