Communism throughout history has been a failure. The Soviet Union was communist and failed, as a matter of fact, even communism in present day country's, such as China, is deteriorating. Communism practiced by modern nation-states has been a failure and continues to be currently.
One major reason why communism has failed in the past and is failing in the present is because communist governments can never be completely communist because communism really shouldn't have a leader. Actually, almost every who has tried to have a communist government country (Cuba, North Korea, The Soviet Union and China), ends up with a totalitarian government. This creates social and political instability because the people have no civil rights, no privacy and even th religion is controlled. This caused the people to revolt because they aren't happy. If your people revolt, your country's government is going to go downhill fast.
Many countries feel the need to "butt in" to other countries business. Some countries try to stop communism/totalitarianism in countries because they feel it would be in the best interest of the people of that country. An example is how the United States is invading Iraq and changing the government because we feel that it isn't a "high-quality" government. It is indisputable that if a country is trying to change your government and uses force your government will fail.
Finally, and probably most importantly, communist governments can't compete with other countries as far as money is concerned. One reason is because most countries won't trade with them. Also, people wont work hard if they don't get rewarded. For example, if a Doctor is getting paid as much as a Garbage Man, the doctor wouldn't work very hard. And if a person who works 4 hours gets paid the same amount as a person who works for 12 hours, no one will have ambition. This causes society and economy to stagnate. There will be no technological advances because desire to make money moves technology forward.