Probably because communism leads to overpowering governments and peoples freedoms being taken away. Adolf Hitler commited terrible atrocities - we don't have any facist states any more. Joseph Stalin commited terrible atrocities - Countries and peoples learn. Look at China and North Korea for example - You can't have more than one child, they get fed propoganda by the bucketload. Its better to live in a state were you can have the freedom of speech, choice and the country is relatively stable. China, North Korea, Cuba see many people living in poverty and have bad diplomatic relations with the big 'democracies' like the USA. This is why democracy has flourished.

Communist states always seem to end up with a brutal dictator. look at Iosef Stalin or Kim Jung Il or Jiang Zemin instead of a comittee which can cover all social aspects

First of all, I would refer you to the answer that I gave to the question on the difference between Marxism, communism, and socialism (http://www.answerbag.com/q_view.php/1903). In that answer, I explain why communism doesn't work economically. To put it simply, communism takes away the incentive for people to work. Just as an example, under communism, the Soviet Union was a net importer of food. Despite the fact that the country had plenty of farmland, their farmers just did not produce enough to feed the nation because they got no more benefit from working hard than they got from doing the minimum work that was expected of them. When the Soviet Union collapsed and the free market took over, the farmers could suddenly earn more money by growing more food. This inspired them to work harder. Now, Russia is a net exporter of food. The farmers are producing more than the people of that nation can eat. So, they now have a surplus to sell on the world market. Second, as others have pointed out, attempts to establish communist societies have always led to repressive regimes that seek to control just about every aspect of the lives of the people. The result is a loss of freedom. People are not allowed to think for themselves. A prime example of this can be seen in the geological papers that were coming out of the Soviet Union. Because the top geologist in the U.S.S.R did not believe in the theory of plate tectonics, just about every paper on the subject that came out of that country tried to disprove the theory. All of the other geologist there knew that they had better agree with the top guy if they wanted to be able to keep working in the field. Such an atmosphere of fear does not bode well for advancement in any field. So, communist societies tend to stagnate and fall behind those that allow people to think for themselves. These are a couple of reasons why communist societies don't flourish.

Freedom of thought is the simple answer, but it goes much, much deeper than that. Democracy has it's flaws as well and eventually democracy too will fall in favor of something else. It won't happen in my life time, but democracy has only been popular for a short time and is in no way fair to everyone. It's far more fair to those with plenty of money, but poor people are left holding empty bags. The thing that will have to evolve will have to be economic theories. Politics aren't the same as economics even though they usually control it.