I have a theory on this, maybe others have some thoughts.
I would bet that many of the "pro-democracy" supporters overseas are local-born Chinese millennials. I would bet extra that they have almost no knowledge of Chinese history and are also not literate at all in Chinese.
I must admit, I am not fully literate myself, but I know enough to read a menu, headlines, that sort of thing. I suspect that the lack of knowledge means that they do not fully appreciate being "Chinese". To put it simply, if you aren't interested in your own language, its easy to not care about the real history (and not just the eurocentric version).