Yes, labor unions is one key factor, but it goes beyond just the employment angle. Over regulation in general acts as a significant headwind against any material renewal of manufacturing (at scale) in the United States.
If you are interested, my favorite example has been the ongoing class action lawsuit against TSMC and its Arizona chip plant. The lawsuit raised various issues including employee discrimination, a hostile work environment and safety concerns.
This is a foreign investment project having near full support in both Washington and among business interest. Still, TSMC has found itself wrapped up in unhelpful legal challenges. The overly litigious nature of America is typically highlighted as the primary reason why foreign businesses refrain from making large investments.
Have a read of the article below. The fact that the case hasn’t yet been thrown out by any judge indicates, at least to me, that foreign business will continue to tread carefully. The Trump team may very well continue to focus on large, headline numbers of committed investment from foreign investors. How much of that investment actually lands, well, that is a separate issue altogether.
https://www.abc15.com/news/local-news/investigations/former-tsmc-employees-speak-out-about-discrimination-hostile-work-environment-at-phoenix-chip-plant