I think democracy is perhaps a fundamentally unstable system. It requires constant corrective force, and a lot of it, from a lot of people. When the members of that democracy start to get a little too comfortable it fails.

Humans are designed to operate in smallish clans with benevolent dictators whose right to rule is based on social currency with people they personally know. Anything else is a house of cards on unstable foundation.

I'm not sure how you fix that, but democracy so far has maybe been the least bad patch.