See the following articles for the historical context [1][2]:
"Even in the '60s and '70s, the Foreign and Commonwealth Office in London attempted to introduce democracy and free elections only to be told by Beijing, including by Prime Minister Zhou Enlai, that under no circumstances would [China] tolerate a democratically elected Hong Kong because they saw that as the first step toward independence," she said. [1]
"These documents … show that not only were the Brits mulling granting Hong Kong self-governance in the 1950s; it was the Chinese government under Mao Zedong who quashed these plans, threatening invasion." [2]
[1]: https://asiasociety.org/new-york/why-didnt-britain-democrati...
[2]: https://qz.com/279013/the-secret-history-of-hong-kongs-still...