What were the causes for the demise of Communism from 1970-1991??????

Communism is a system in which the government has near complete control of every aspect of society including the economy. This creates dependency and puts absolute power into the hands of a few bureaucrats. Absolute power corrupts absolutely so communist societies are usually run by corrupt politicians or tyrannical dictators like in North Korea. There is no incentive for the people to produce anything in a communist society because there is no payoff. All of your work goes to the benefit of the "state" so no matter how hard you work, you receive no better payoff than someone who works half as hard. This creates an unsustainable system where everyone depends on the government for their needs, but the government has no resources to meet them because the people are not producing enough goods and wealth. Eventually, either the economy collapses and the nation follows, or the people get tired of living in squalor and civil strife ensues.