The reason Taiwanese Aboriginals today predominantly live in the highlands is that, when the Han Chinese settled Taiwan, they killed and expelled the ones in the lowlands. For hundreds of years, the Chinese in Taiwan killed Aboriginals. They even ate their flesh, believing it had medicinal properties:
https://zh.wikipedia.org/wiki/%E7%95%AA%E8%86%8F (use Google Translate). As you can imagine, this bred resentment among the Aboriginals, who therefore vote for the KMT against the Benshengren-dominated DPP.