The general consensus seems to be that human nature is the problem. But as an academic in political philosophy the problem is the makeup of Leninist/Maoist/Stalinist Communism is the shear level of Paternalistic/Authoritarian control existing and past societies have imposed upon their people. Carl Marx wrote that societies need to naturally progress to a more Socialist outlook through peaceful reform, he effectively says that Capitalism is the first step towards socialist prosperity.
It is important to bear in mind that when we talk about Communism, that like Liberalism/Conservatism, it can take many forms, and in fact has been proven to work quite well within a countries that have a multi-party system with Democratic elections, While maintaining an essentially Socialist economic structure. Sweden, Iceland and Denmark come to mind here.
To say why Communism in its Leninist forms failed so miserably, is merely based on human nature is quite a simplistic view. While human nature may have played a part in the failure of the Commintern Project is quite correct, greed over power, wealth and social status certainly played a part, despite claiming to have demolished social differences, the Commintern Project failed spectacularly in breaking down class segregation and jealousy, it failed to recognize that while economic status may play a role in jealousy it is not the only cause, and this is essentially the problem. Thus essentially the project of Communism caused its own failure by failing to recognize that human have needs, and that while all human may be morally, and socially equal under this system, you still have those who work harder than others, those who are lazy, ect.
Here is a thought experiment for you:-
Suppose you live in a communist system where you and your neighbor have the same size plot of land, the same level of capability in working that land, same state income, the same size family.
Now suppose that your land produces enough food to feed your family and the government takes the rest of your produce, while your neighbor produces a fraction of your produce, can't feed his/her family and is constantly borrowing food off you year in and year out, But still this person gets exactly the same reward as you, you get no recognition no thanks.
How would you feel living in such a system?
While i agree that Socialism/Communism makes some great points ideologically, in practice to date the system has failed not merely because of human behavior but because of the way in which it has been implemented in the past. There are more ways to reward someone than simply financial reward. Employment promotion, government promotion, promotion in social responsibilities leading too a better life for individuals and their families. Essentially a meritocratic approach to socialism.