Three big reasons:

1. Capitalism thrives on the promise of social elevation. You can become billionaire too, you know, you just have to put in the work and be smart. And many are willing to sacrifice their living standards for the hope of making it big, however unlikely the latter is.

2. Capitalism is a current economic system in the majority of places, and most people are afraid of the change. They'd rather maintain the status quo - this is why, for example, socialist countries had majority of the population supporting socialism - not because they got enlightened to its potential, but because they already lived under it.

3. Privately owned media. Most people get their information from the corporate platforms, which, being owned by wealthy individuals benefitting heavily from capitalism, obviously downplay the horrors of capitalism and exacerbate issues of socialism. It is them who propel the "hope" narrative (see point 1) and thwart any serious attempts to popularise socialism. Even if you manage to host socialist media on socialist platforms and outplaying the algorithms, good luck getting corporate funding to make quality content with high level of reach when your platform is anti-corporate.