Because the wealthy who push propaganda in both structure and content down our throats 24/7 do not benefit from socialism.

Because they don't understand either one.

It started with the cold war, though, when Communism became the boogeyman.

Well, maybe tankies hadn't warped the idea of socialism, broken it beyond any meaningful value to the working class and turned it into surrealist, blood-drenched terror the working class wouldn't be so damn afraid of it.

I think it's more to do with that no one questions how horrible fascism is

I think it's very easy to just point at propaganda or a lack of effective outreach, but I think a big part is that we're a low-trust society, and most people have a hard time believing that they won't be excluded from the collective, because that's not our experience elsewhere in life.

Also, fascism doesn't call itself fascism. So the real question as most people perceive it is, "why is socialism scarier than conservativism". We know it's actually fascism, and thus know to fear it, but most don't.

Lastly, I think the false narrative of meritocracism in capitalist/ fascist societies plays into the psychological bias where humans tend to perceive themselves as average or above average, and makes people vote for rigid hierarchy (where they all think they'll be closer to the top than the bottom), against their own interest.