But what are the reasons? You can claim the idea is a bad one only if you can attribute the failures to the idea itself. Because I don't see that way, I can't say the idea is bad. I see a lot of human mistakes.

Also the final win-out of capitalism is somewhat by chance too given the advantage the US has had after the WWII. Of course along the way they quickly corrected their mistakes has been crucial here too. But this only proves democracy is better, not the capitalism itself.