What was adopted was an adoption of existing encodings mapping, as per rountrip convertability policy. If Hangul had a working composable encoding, it would've been used instead.
The problem is that Hangul had too many composable encodings from each vendor. As a result the government went to yet another standard (KS X 1001) that fits better to the ISO/IEC 2022 infrastructure. It was too late when the standardized composable encoding was specified as an annex to the original standard in 1992: Windows 95 didn't care about the annex and introduced their own extension to KS X 1001, now known as the code page 949 and standardized in the WHATWG Encoding standard [1].
[1] https://encoding.spec.whatwg.org/#index-euc-kr