Here's three possible answers, all of them a little bit flip:
1) They have more guns than we do. Especially in the US, which is the history I'm most familiar with, the government has this neat little trick of killing off the opposition. It's amazing what few strategic murders and intimidation will do.
2) The have more channels of communication than we do. How can you disseminate democratic ideas without a democratic media?
3) Capitalists have nurtured a strong sense of internationalism. We should do the same.
While I do think a little soul-searching on the left is always a good thing, I also think it's foolish to blame ourselves when we're clearly the underdog here. The sectarian namecalling and blamethrowing is largely, I think, anger turned inward that should rightfully be turned back out. I think if you ask basic questions about right and wrong, most self-identified leftists would agree on the answers. The challenge is how we get to right from wrong.
Best, --J