While any answer is going to involve some share of opinion, I think Trump’s popularity on this issue comes more from a general sense that he’s willing to stand up to China, rather than any of his policies that specifically benefit Taiwan, which have largely been continuations of those of his predecessors.
In Trump’s favor, his trade war has significantly cut trade between the US and China, putting pressure on their economy. Many have rightly criticized previous administrations for talking tough about China’s human rights abuses, but then undermining their message by continuing to trade freely with them. If you see China as an existential risk, Trump’s willingness to put tariffs on their trade looks like a real effort to stand up to them.
The counterpoint on this is that, since the trade war is about “trade fairness”, not Taiwan or Hong Kong, there is no reason to think that the pressure will result in anything but benefits to the US. In fact, since a real trade war would hurt the US economy and Trump’s re-election chances, his need for a victory here may have caused him to de-prioritize other concerns, such as when he promised not to speak out about Hong Kong as long as trade talks were progressing, and his reluctance to sign the Hong Kong Human Rights and Democracy Act:
Last Friday, in an interview on “Fox & Friends,” Mr. Trump said, “We have to stand with Hong Kong, but I’m also standing with President Xi.” Last June, Mr. Trump promised Mr. Xi in a telephone conversation that he would not speak out in support of the Hong Kong protests as long as trade talks were progressing
Another point to consider is Obama’s negotiation of the TPP (which Trump withdrew from) which would have also put significant pressure on China through a multilateral trade deal with their competitors in Asia. While he doesn’t get credit for it, since it was never ratified, the TPP might have had put similar pressure on China.