(1) The talent pools in Taiwan and China are different
(2) The markets in Taiwan and China are very different
Right now the Chinese market is more important than the Chinese talent pool, but both are important. Basing an accelerator in Taiwan would be good for Taiwan, but it would not meaningfully engage with the Chinese market or talent pool. It's too late in the game for that.
Even if YC's ultimately goal were political and humanitarian it would still be more effective to base the accelerator in China and use the additional leverage gained from being in China to better pursue those goals.
Disclosure: I am an American currently living in Taiwan. (edits: grammar)