“I do not understand why, after Stalin, Russia allowed Putin to become a dictator; nor why xxx, after xxx, allowed xxx to become one.
Both countries were invited with open arms to join the Democracies of the world – and both declined.”
（The answer in English was attached in the second half of the post.
It is a large topic, let me try to share some of my views.
The authoritarian system has succeeded in several things. First, they have managed to make their top 10-20% - politician/elite/rich/powerful classes, i.e. those who have the most sources to bring a revolution or turn the system upside down, believe that the direction of the country will be more open/outward/domacractic/free and the current non-free phase is just temporary and necessary "sacrifice/endurance" to a better future. It is not urgent for these people to make change. Freedom is a privilege/specific interest in authoritarian countries, however, the culture there turns it as a motivation to make people want to climb the social economical ladder achieving individual and or specific interest group's success, rather than fight for the general public's freedom and equal rights. Of course we can always find individuals contradictory to this, however, I talk about statistics here.
Second, the system has used nationalism/tribalism to make the bottom 20-30% believe that democracy is a western branded thing, therefore, it is like a "culture invasion". If they adopt it, it is a treason to their ancestors and traditions. They also fear that there would be a war going through the whole process - changing from the current system to a democratic system. As long as the current life is still tolerable, not enough people would be interested in taking risks. These BS and fear mongering propaganda indeed work well.
Third, the authoritarian system is a pipeline, there is not only one dictator kind of person, but tens of millions of them. They control (or want to control) people all the time in daily life, work, school, home, sports, internet, everywhere.
Fourth, the high pressure and propaganda machine in an authoritarian system have effectively destroyed people's independent thinking and viewpoint diversity. It makes a huge amount of people view the world either black or white. Even if all of a sudden people get the right to vote, there is a high risk they would vote for a new dictator. That's one of the reasons why regime change wars have failed in many countries. It takes time to practice and build a healthy democratic system. Democracy is still a very new culture, society and political phenomenon compared to the entire human history. If we look at evolution, a hundred years is too short compared to homo sapiens' whole evolving time. 99.999% of human history is under dictatorship. We are basically fighting hundreds of thousands of years of human history. It is similar to the case that we all know high salty foods such as pickles are not healthy, however, people still cannot resist those foods due to our long history of starving. Even the modern democratic system has not been accomplished in one day. It had budded thousands of years ago in ancient Greece and Rome, and in the recent century there were still mistakes such as the German people electing Hitler.
Last but not least, we often believe transforming from an authoritarian system to a democratic system is moving up the social/ political food chain. I personally believe so. However, it could be a process of hundreds and even thousand years in some countries. For a generation's length of time, it is more practical to treat non-free society and free society as "diversity" of political/social systems. In other words, let's stay away from their own troubles but focus on strengthening America's own freedom and democracy first. For instance, there are generally two types of species that can thrive on an island. One is birds with strong wings and muscles, the other is insects without wings or half-wing. Should we install birds' strong wings to insects?
BTW, my understanding of the USA is not a democracy but a Republic, which is another big topic, to make it simple please refer to the attached image.
That's my two cents.