In order to understand why, we must first understand the group behavioural dynamics of individuals which could be best explained using the example of 'the prisoner dilemma'.
Assume there are 2 prisoners X and Y caught by police for a minor crime which could land them in prison for a maximum term of a month. Police however believes that these 2 are also involved in a more serious crime as well, but there is no evidence, so police must strategically get them confess. Police decides to interrogate them individually in separate rooms and puts an offer for them. Each convict gets a choice to either stick to the minor crime in which case he is bound to face a months term or to defect against his partner for the bigger crime in which case his partner will have to serve a term of 1 year and he will be set free. But each prisoner is aware that the other must have received a similar offer. So there could be one of the 4 outcomes now. 1. They both stick to minor crime, 2. X defect agains Y but Y remain loyal, 3. Vice versa of 2 and 4. Both defect. Now as a group it is in their best interest that each keep sticking to the minor crime cos they know there is no way police could ever prove the charge for bigger crime and thus being loyal to their partners or the community they belong to, they both or the community will be better off, each serving just a month, but here is where the theoretical, on paper logic starts to drift away from the implications of the practical implementation of the concept. In most cases an individual human being is hard wired to put his own interests above the interests of others and in this particular case, there is also a threat attached as a repercussion of being loyal to the community cos there is no guarantee that ones loyalty would be reciprocated in a similar way. This when couples with yet another set of complex parameters involving human thinking and decision making becomes even more difficult for communism to prevail.
Prisoners dilemma had just 2 key players and yet the situation had just 1 in 4 chances to have turned out in favour of community. Imagine a similar situation with a larger population of millions of people, the probability of things turning out in community's favour would be dramatically low in that case.
