I could see another probable reason for this. The disparity between how popular learning English is and how few native-speaker teachers would be available to hire means that demand couldn't be met by the supply, so it would cost the government a fortune.
Out of curiosity, what does foreign language education in public schools look like in Taiwan? Here, it's treated almost entirely as a school subject rather than a practical skill, so there’s lots of theory, tests, and focus on written English, while practice, drills, and spoken English are almost entirely ignored.