From https://www.nytimes.com/2020/05/14/technology/trump-tsmc-us-...

Mark Liu, T.S.M.C.’s chairman, told The New York Times in October that T.S.M.C. had been negotiating with the Commerce Department about a possible U.S. plant, but said it would require substantial government subsidies. How much, if any, public money might be associated with the company’s decision was unclear.

And from https://www.bbc.com/news/world-asia-48917705

The US State Department has approved a potential arms sale to Taiwan, estimated to be worth $2.2bn (£1.76bn), the Pentagon said.

Maybe TSMC thought it would be really good if Tawain were protected better thanks to these weapons, and TSMC would also like these subsidies.

Taiwan cannot afford to lose the support of the US arms industry.