It's because of social media. It creates echo chambers. People prone to hate, greed, or just stupid people, tend to flock to certain echo chambers. I think it's by design. Silicon valley wants this right wing dystopia.

Propaganda works and the rich own every popular media ...

It's not that it's especially effective. It's that the wealthy have been consistently pumping money into blaring division and individualism into every corner of society they thought relevant for decades upon decades. They've had plenty of time by now to figure out enough of what does and doesn't work for them (including types of journalistic reporting to promote and suppress) in terms of manipulating the news, and promoting division in many ways (including not just stirring up controversy about social issues, but other forms of division as well).

We were never going to outspend them, so I think our best bet was always to out-human them. (And probably to set up parallel economies if things get really bad.) For now, we can still start at the bottom of Maslow's Hierarchy and meet people we want to reach where they're at. If they aren't surviving well, they're not going to be as focused on other issues. Wherever they're at, we should focus on listening and empathizing first, then poke holes in the lies they've been told once they feel heard and respected. People who have been where they've been are the most likely to reach them. To me, all the MAGA regret we're seeing now in the US, and all the chemical spills and natural disasters that don't get major media attention and which local governments are too underfunded to help with are an opportunity for somebody to reach some of these people (ideally with monetary aid from an organized movement), so they can get their heads right and get to work on waking up those to whom they're adjacent.

Simple why it's so effective:

Institutions are facing increasingly insurmountable problems, and people are not dealing with the consequences and stress that comes from these problems. They want answers and solutions.

Institutions ask for you to have patience and give them time to fix the issues. Problem: people already don't believe that the institutions are fixing things, so you can't rely on them to fix it. The solution, they feel, has to come from elsewhere.

Left-wing propaganda says that the institution is broken, they will fix it. They want to take the money away from the ultra rich and give it back to the public as a form of equalization. If you are part of the ultra rich, you don't like this.

Right-wing propaganda says the institution is broken, they will fix it. Right-wingers generally believe in the accumulation of capital for themselves via "merit", which means the upper class and billionaires are likely to be supporting / using them.

People believe that things need to be fixed (which is likely true). Only left-wing and right-wing propaganda say that the institution needs to be fixed, it definitely isn't coming from the institution itself.

The ultra rich fund whichever propaganda will allow them to retain their own benefits and works to suppress the propaganda of the ones who work against their interests.

Hence, right-wing propaganda is massively inflated to support fixing a system that allows retaining wealth, while they suppress left-wing propaganda which says wealth needs to be redistributed to the masses.