Answer:
Because of the overpowering status of Capitalism.
Explanation:
In Marxist and Communist Thought or Ideology (sometimes the same thing), there are many different perspectives on the matter. The Classical Marxist thought would say that Communism never truly existed, and cannot be a regime. False regimes have come up under the guise of Communism (but truly monarchies without religion), but they were truly supporters of the Capitalist cause (which isn't as much an unjust cause as it is a necessary cause). Without Capitalism, true Communism would never come around.
Marxist-Leninists might say that they disappeared because of rightist agendas of some members of Communist parties. Or they might say that Communism has not died out, but lives on in places such as DPRK (North Korea) and Venezuela or Cuba.
