China broke its word and trashed the deal. Eventual certainty notwithstanding — the CCP demonstrated (as it has oh so many other times) that their word is unusually unreliable, even in the shitty arena of international politics.
HK relies on Chinas word. Taiwan doesn't.
That's pretty different to Taiwan which has never been under the sovereignty of China.
You see, if China agreed with your statement diplomacy would be a lot easier. But few things anger the Chinese more than asserting that it does not have sovereignty over Taiwan, the only other one I can think of is that Tibet should be free to be its own country.
But Taiwan has a functional, large military, friends in Washington and economic consequences to an invasion.
HK had umbrellas and a few strongly worded newspaper columns.