I recently read the book The Dictator's Handbook, which discussed this topic among others. It makes the argument that for such a movement to succeed, the existing regime must be destabilized in a way that makes its key supporters lose confidence in the regime's ability to continue to reward them. Often this is because a leader is sick, or the economy collapses and the leadership runs out of money. When that happens, those key supporters hedge their bets. Army generals refuse to fire on protesters, or local officials refuse to disperse them, etc...

In the case of Hong Kong, clearly Carrie Lam never lost confidence in the PRC's ability to keep her in power, so she and the other loyalists in the HK government never put up meaningful opposition.