Afghanistan never had an effective democratic government; Iraq may have one now.

> ... Iran, China, and Vietnam are all examples of this failure.

China, and Vietnam never had democratic governments or anything like them. Iran has had some semblence of one, but on a limited basis (ultimate power lies with religious leaders, who can even ban people from running in elections).

But parts of China have had very successful democratic governments - in Taiwan currently and formerly in Hong Kong.

> there’s clearly something different about all those places that it didn’t quite end up the way the rest of us would consider acceptable.

Why is there something 'clearly different'? There are many explanations. All the evidence we have is that people in China love and preserve democracy whenver they can.