Like most of the recent rebellions, they expected too much of the west, especially of Britain which promised to keep China at bay (official statements of a Prime Minister is a promise).
Actually, I'm fairly certain the UK promised to make sure that for 50 years that Hong Kong would be allowed to retain their autonomy. To which China has not reneged on so far. And even if they send in the PLA to crush any dissidence, so long as China doesn't do anything else to the structure of Hong Kong that affects its autonomy, it's still technically staying true to the agreement of 50 years given to HK, so I'm fairly certain no one is going to go head to head with them over anything, especially over a domestic issue.
Unfortunately for Hong Kong's resident democrats, they lost in 1997 and nothing is going to stop Beijing rolling in before or at the nominal 2047 'integration', they might as well accept it or apply for asylum.
This was always going to be the case. Many fled during 1997, I know many of them here in the UK who were able to leave back then. But as for whether they can really apply for asylum is a grey area I feel. If asylum was to have been given, it would have been granted back in 1997 already and many more would have been able to leave. But clearly they didn't get granted back then, so I don't see why they would start to now. Also, where will they go? You're basically talking about the whole of HK virtually that doesn't want anything to do with China (considering familial links with people who want to leave), and with a population of around 8 million. Assuming lets say that only half (a generous number considering what is known) wants to apply for Asylum, that's still 4 million people. That's not a small number. And all this over currently still unproven issues with China; again, what China promised during the handover, technically they have not reneged over. I don't think any country is going to grant mass Asylum to 4 million people; individuals and small numbers maybe, but half of HK? No way.
Again, as I said much earlier on, the people should have been working to get as much for themselves as possible instead when they had the momentum, but now, they've only dug themselves into a hole they can't get out of. It's basically the same thing in the Civ games: A Mutual Defence pact means if someone outright attacks or declares war on you, I'll join in. But if you, the other member of this pact, force someone else to attack you through attacking them first (not the same, but you get the idea), I'm not coming over there to defend you; It your own issue. Treaty or no Treaty. You don't go all gung-ho and incite problems, and expect me to solve it. Same here.