Communism failed because of corruption. I used to be a Marxist myself, until I got off my high horse (when I actually went and looked through Soviet history), and realized that every society's downfall is due to the basic human instincts we all have: greed, envy, fear and decadence. Because the majority of people have at least a little bit of these traits (myself included), it is only natural and people who get into "power" over others become drunk with their own success and proceed to screw over anyone they please, simply because they can.

The utopia versions of the two main political ideals, Communism and Capitalism, are often what people automatically think of when the associated word is spoken; when a capitalist hears the word "capitalism", one would often picture a bustling New York scene with lots of money being moved around, people going to work, business owners and a strong economy with the freedom to choose where you work and how you spend your money. When a communist thinks of the word "communism", they would often think of a society where technology and innovation is slow, but the economy is sustaining. People would receive their food, property and such as long as they did the work that was assigned to them. General good-will to others, and the idea that your work is helping less-fortunate individuals become apart of the system and lead a happy, honest life.

In reality, neither of these visions is correct. As seen with the present Western Capitalist regime, corruption and greed has morphed it into a massive imperialist conspiracy, with the business owners buying and selling slave-labour on wholesale, giving their workers $7.50 an hour for retail and $10 an hour for hard industrial labour, and making the governments bow before them and instate laws like the DMCA to ensure that they control the freedom of what makes capitalism, capitalism. The money goes up the food chain consistently until there is a handful of people in control of 97% of the world economy and resources. This is what Marxists actively refer to as "class strugle", where the worker has absolutely no chance against The Mighty Capitalist.
In a Soviet Communist perspective, the corruption was apparent. Instead of the government being this benevolent force in the universe, ensuring that everyone could make ends meat, the government was so easily twisted and killed by Stalin. If you failed to meet an unrealistic quota, you were shot. If you met the unrealistic quota, you were used as a propaganda figure, and then shot out of spite. The main problem with the Soviet Communism is that it took Capitalism and gave it a sadistic socialist spin, which would be like burning the constitution of America and giving the government total control over all assets in the country. Lenin, after the revolution, actually realized what a massive fuckup he had commited, and then tried to rectify it by opening up the markets and trying to fall back on a kinder, friendlier socialism. Of course, before he could do anything, Stalin took power (Lenin himself said Stalin would be the death of the Soviet empire), and it was all downhill from there.

The problem I see with both societies is the similar aspect of people believing one is better than the other simply because of the amount of money they have. Who is to say that any wealthy Capitalist business owner or any Communist government leader is better than anyone else?

In summary, the Soviet Empire fell due to the fact that the style of government was a direct rip of a modern Capitalist government, with a dash of police state tactics. Capitalism will eventually fall due to the current sustainability problems inherent in 95% of all businesses operating tactics.