To me it's simple: capitalism is a more-successful system than others in our current society, and has been for a long time. (Quite possibly due, in part, to the fact that capitalism has helped form our current society.)
I would suggest that taking away capitalism in the world now--or trying to revert to a socialistic system--will remove a lot of necessary ambition and drive in individuals as well as governments, resulting in a less-focused race of humans, thus inherently less technological and societal growth and development.
Past that, right now the evidence is that capitalism has been "the most successful" form of growth/development in economical and technological ways in the past several centuries; what motivations would a government have to switch/change, aside from the uneducated masses telling it that it should?
Putting all that aside, I believe that a form of socialism is ideal for us all; capitalism has many negative side effects, most prominently the aforementioned focus on consumerism. Consumerism alone breeds envy, jealousy, greed, and other highly negative mentalities (albeit most are subjective), and generally has the one positive effect of increased individualism and economic/technological growth.
Do these benefits outweigh the drawbacks? It is my opinion that right now, they do--but only because we are not ready for being more responsible for ourselves; as a race, we are not ready to take ownership and responsibility for others and our own decisions, to act like more than individuals. I hope that in my lifetime we "achieve" this, and can switch to a less "forced individualistic" mindset world-wide. But I'm also realistic. Change like that takes time or massive catalytic events. We will see.
Right now, capitalism breeds individualism, which is the only long-term or "true" good in the system, that same individualism could be applied in a healthy manner inside a non-capitalistic state. If only the people would do that, instead of simply relying on their respective government and their fellow brethren to give them what they need/want, turning away from ambition and growth altogether.