Some socialists try to point out that socialist (or communist, its kissing cousin) policies are "sound". Sure, if you can get people to follow them. They include the policy that govenment owns all industry, business, etc., and gives its workers the housing, food, education etc. they need. The ides is, that they will work for the greater good of society, and keep the businesses flourishing. There are other features of socialism, but these are some of the most important.
But people in a socialist system generally don't want to obey socialist policy. It's a tendency that increases with time. Rather than work harder at something that brings them no additional benefit for it, some slack off while most keep working hard. Later the hard workers notice the slackers, and a few more slack off. Later a few more, etc. The system doesn't collapse suddenly, but merely gets more and more inefficient and unproductive over the years, or even over generations. Then when other problems happen (natural disaster, foreign invasion, etc.) recovery is difficult or impossible when the country has slid far enough into sloth. This is the cause of the repeated failures of socialist economies.
And the only thing the leaders can seem to do about it, is force people to work hard whether they like it or not, whether their pay goes up commensurately or not. In other word, they must rule contrary to the will of the people, rather than in accordance with it. In other words, they must become dictators.
Socialism breeds dictators. But it's not the dictators that bring down the society. Socialist policy, and the disinterest in the population in following it, bring down the society.
Capitalism, with its tendency to (usually) reward hard work and initiative, has it flaws, but that isn't one of them. And so it survives and prospers, however roughly and unevenly, where socialism fails.
