It's important to remember three things:

1. Historical democracy was generally diffrent from modern versions, being either exclusively available to aristocrats, or heavily weighed in their favor; and violence was pretty much always an option for deciding disputes if voting didn't go so well. As such, the difference between a kingdom with democratic elements and one without was often more academic than practical.

2. Historical democracies often gave way to autocracy: Rome most obviously, but even Athens ended up becoming more of an argument against Democracy than for it given the wars it lost to autocrats.

3. Democratic impulses aren't always obvious: China might not have voted for emperors or bureaucrats, but the idea of the public overthrowing non-virtuous ones is at least as old as Mencius (300 BC,) and massive popular revolts are a common theme in Chinese history. It may not be conventional, but then, western democracy was also built on incompetent autocrats getting overthrown by the people doing the actual work, so most of the world may just have never gotten the chance to do this once Europe started playing king maker.

As to why western democracy emerged and ultimately dominated, there are probably as many theories as historians, and the effect of trade is certainly a major one, but so is the branching effect of gunpowder coming to dominate warfare. There isn't really a simplified answer.