The biggest factor is import tariffs
Used to work more to China's advantage (protection from American car imports) but now works to its disadvantage (since American car makers are now protected from Chinese imports).
Assembling a car is < 1 days labour, so even if you're paying high north American labour rates, it's still cheaper than a spot on a ship from China.
Japan used to make a lot of their USA-bound cars in Japan, even the cheap ones. I guess that is a difference between the 70s and today, though.