All American presidents prior to Trump would have defended Taiwan on the basis that it is a friendly liberal democracy facing a totalitarian aggressor. Trump couldn’t care less about that. He identifies with the powerful predator, not its potential victim.
Trump’s predecessors would also have valued Taiwan as a capitalist state with a vital role in the world economy. Trump sees Taiwan’s chip industry and wants it for himself.
That leaves the defense of the sea lanes around China as a potential reason to defend Taiwan. More on that in my next China post.