The US always had the best facilities, biomedical research started in the US the Rockefeller University was the first one in the world iirc.
This doesn’t explain it really what could explain it is that as the cost of developing a drug became higher more and more players dropped out of the game especially as the revenues started shrinking due to Indian generics becoming popular around the world.
Since you don't really need to be in US to create drugs for US market.