The explanation for that is easy.
Most of the people knew with which side to side long before the war started.
No amount of propaganda is going to convince an average west European or American that Russians are good guys who just wanted to protect Russian-speaking people in Ukraine. Moreover, no amount of "western" propaganda will convince someone in a country that was hit by the USA or NATO bombs that Ukraine supported by "the west" is fighting for freedom and democracy.
With propaganda, you can soften these opinions a bit, but that's the limit.
And it just happens so, that both Reddit, Twitter and HN are dominated with the users from the first group, so they tend to believe Ukraine side more.