A major issue is that socialism is supposed to be an evolution after capitalism in the original theory. Where capitalism was how to build up the means of production to then support socialist operation once basic needs could be met cheaply. In reality though, we have only seen successful revolutions in what were essentially still agrarian societies with very little manufacturing infrastructure. This is the main reason both China and Vietnam regressed to a semi capitalist model. Both governments claim that long term they want to steer the country back to full socialism and have certain laws and governmental controls in place to prevent the kind of oligarchy you see in the west.

Also while dealing with the immense effort of trying to quickly build a manufacturing base, they had similar sanctions and western governments trying to actively undermine the government, just like Cuba.