1. A local government friendly to the US. Gov type did not matter.
2. US Workers starting to want medical benefits, by the late 60s, hardly anyone would take a job without benefits. This includes sick time.
3. Environment Controls starting in the 60s. I think this is a big one.
4. High Wages in the US
5. Improved intercontinental flights and cheap shipping. I think in the 60s the US merchant was quickly being outsourced to cheap countries.
6. An of course corporate greed and the rush to make Wall Street happy.
You just need to look at ROC in the 90s, lots of companies started moving manufacturing there because Japan and Taiwan was getting too expensive.
Now I think they are slowly moving out of ROC to places it SE Asia like Vietnam.