Crimea has the problem that the annexation was very, very popular among Crimeans, even more so than most separatist movements. That doesn't make it okay, but after the fact, it'd be hard for third parties to justify forcibly reuniting Crimeans with the Ukranian government they don't want to be under.
It’s a bit reductive to describe Hong Kong as a “Chinese territory.”
What do you think would happen if China attempted to forcefully annex Hong Kong?