Interesting question which I often think about myself, there are various factors that contribute to this in my opinion:
1) Most of these countries were not really communist (sorry...) they were thinly veiled elitist dictatorships which gave 'Communism' a pretty bad name.
2) We live in a world of finite resources, progress in terms of standard in living for the richest capitalist countries is based entirely on the exploitation of poorer countries in order to be able to sustain expansion of economies and in turn ther own standard of living, so although I agree with much of what you have said, I think that BELOW the communist countries in terms of SOL would be a huge amount of exploited third world countries (see sub saharan africa, and much of Asia), without those the Richest capitalist countries would not sustain their development.
3) Again living in a world of finite resources communism is always going to lose against capitalism, the nature of capitalism is growth and of a predatory nature, communism is all about (in theory) internal balance, sybiosis and equal distribution (although this was not entirely the case as most communist states were basically dicatatorships).
As an analogy, imagine you put two species in the same environment, one which constantly devoured resources, hoarding them for themselves and preying on the weaker members of the environment raping and pillaging at will, then imagine a second species who just lived in harmony with their natural suroundings, what woud happen? I know this is a clumsy metaphor, dont bother picking it apart as it is just meant to highlight a point, not be a literal interpretaiton of events.
'Communism' never stood a chance against the nature of capitalism, real communism would stand even less of a chance. The only way it would work would be on a world wide basis run by a benevolant and fair minded dictator, the Dalai Lama for example (although he would not touch the job with a barge pole I would guess!) It would inevitably fail due to mans inherent stupidity, greed and lack of compassion for fellow humans.
This leads me on to another thought, is capitalism the most perfect system for us because it most closely reflects our natural human instincts (quote Hicks 'we are a virus with shoes') or is mans nature like this becasue we are born into this system then moulded by it?
Whatever, back on topic, that David Cameron is a right nob-jockey