1) Trump might be alienating his traditional allies and cosying up to Russia, but he still apparently sees China as a problem or adversary.
2) Thinking purely transactionally, the US is very dependent on Tiawan due to TSMC. Most of the US' largest tech companies are investing heavily in AI hardware (TSMC chips) and/or rely directly on TSMC for their own supply chain. I have no idea whether Trump et al see it this way, or this would be enough to trigger the US to protect Tiawan, but transactionally, it's immeasurably more valuable to the US than Ukraine.