Comment by monocasa

It's geopolitical. Taiwan is ultimately reliant on the US for defense from China. The DoD wants to maintain close to leading edge chip production on US soil where they feel like they can control the physical chain of security of the chips they use. TSMC builds a fab on US soil, and Taiwan and it's sovereignty wrt to China is better embedded in the power structure of the US defense strategy. The DoD footing most of the bill makes it an easy win.