Presumably because the United States is BMW's second-biggest market, with 40% more sales than in Germany, and so they won't want to get caught up in another trade conflict. BMW is also the largest vehicle exporter by value in the United States, and paying tariffs to import assembled batteries from elsewhere, and then paying tariffs again when exporting the vehicle doesn't make a lot of sense for them.

And, of course, South Carolina workers are more acquiescent, and the state government much less interested in labour rights than other jurisdictions. BMW also gets obscene tax benefits from the state, because South Carolinians love nothing more than giving corporate handouts.