Protests usually follow some egregious government policy or action. Yes, there are protests that aren't very well organized, or lack a core concept to them. But those quickly fall apart with limited or no intervention. When they don't fall apart, it's an indication of something that's already wrong, rather than something that's about to go wrong.
There were actually some leaders with sympathetic and liberal movements in the times that led up to (and during) the TS incident, but afterwards the Chinese government either forced them to toe the party line or removed them from power. Their unwillingness to allow peaceful dissent, their massive overreaction, and their blatant suppression of the facts and of the expressions of grief afterwards have certainly fixed the governmental direction for quite some time. Hardline censorship and oppression supporters got more power, while those opposed got purged or learned to stay quiet. The government policy was at a crossroads until then, but they doubled-down on a surveillance state.
A government that straight up murders citizens during a peaceful protest and then refuses to let them talk about it for decades after has a huge effect. It's not just on the leaders in power, either. Even if they don't get the whole story, the Chinese populace is quite savvy about what can and can't be talked about. Most people will stand up for what they think is right when the risk is relatively small. TS reflected that, with academic elites being openly honest about their opinions about the state and future of the country. China was changing a lot and they thought there was finally a chance for a dialogue. But it was made quite clear that non-violent protest will be squashed and you will quite possibly be murdered or smeared for speaking a contrary opinion - even if it's out of patriotism.
It drove all dissenters into hiding or out of the country, leaving an empty playground for military and political power seekers who had no problem with heavy-handed retaliation.
> Curious how the extreme violence of the Chinese response in Tienanmen is responsible for the continuation of the government politics; while the same extreme response in Syria would be responsible for tearing apart the country.
The thing in common is that, rather than listen to their citizens, the government chose suppression and murder. The citizen reactions in both cases are hugely different. But the source of the problem isn't different - an oppressive government. We don't shame people who back down when the alternative is death or civil war. It's an understandable choice. But we do shame people and governments who literally can't tolerate dissent without military might. Especially if they won't let you talk about it afterwards. Resorting to military might is a sign of political weakness, not strength.