Basically for the same reason capitalism is failing. No communist government has been legitimate. Communism is not inconsistent with democracy, but it's never been tried in a country with a pre-existing tradition of democracy, so the people in these countries just ended up trading one set of despots for another set. The fact is that the great majority of revolutions work that way. The US is exceptional.
The best evidence is the USSR. They abandoned communism nearly 30 years ago, but their govt. is no less corrupt, no more democratic than it was under communism.
Now we're seeing the same thing in the US. The Trump administration AND the Republican-controlled Congress both seem to see the Constitution as just optional 'guidelines'. The ONLY thing the govt has done in the last year was to pass a big tax cut for gazillionaires, the one thing their sponsors wanted. We are now circling the same drain as the USSR.
I think communism could work, just as capitalism could work. It depends on who's running the game, on the people in charge, their integrity, their competence, their goals and objectives.