Because American built cars couldn't match the quality of Japanese (and German) cars.
In my undrgrad daze, I had 3 qtrs of Statistics.
In my MBA grad school daze, I had 2 qtrs of Statistics.
One name always stood out - Edward Deming.
He attempted to persuade Detroit to use Statistics to produce higher quality cars. They ignored him.
He went to Japan who adopted his statistical theory of quality control...and the rest is history.
More reason to thank Musk now.
Tesla’s currently consistently rank highest in customer satisfaction among cars sold in the US.
As well as having most US made parts.
GM let quality slip. And poor designs too. Remember the Chevy Vega? Think back to the first Honda Civic you saw. It was so small it was a joke. Honda is not a joke anymore.
Prices Japanese cars made a killing in the late 60’s.
Detroit production costs caused a huge drop in sales big time.
So the Japanese and Germans matched ours......
I wonder if Detroit experienced the same thing I witnessed in Chicagoland. The Tool & Die industry depended on people who could do high school math and figure out where to put the hole in the metal. In the 1950s and 60s most of those workers were immigrants from Europe. Lousy English...but good at geometry.
Increasingly in the late 60s and especially in the 70s and 80s American kids could not do math. (Sec of Education Lynn Martin & NEA sweetheart came from Rockford, IL... the Tool & Die capital of the world. How ironic.)
The result was that the tool & die jobs went to Asia...first to Japan, Korea then later to China. The lesser skilled manufacturing jobs in Chicagoland then followed the skilled jobs to Asia.