Source: 📖 21 Lessons for the 21st Century ch3
Democracy became the best system for ruling as it spread decision making across many different competences. This allowed experts from each field to contribute to the decisions that pertained to their fields. By contrast, dictatorships like the Soviet Union that centralised all knowledge and decisions fell behind democratic systems, as all that information is too much for one central party to analyse. Of course, this changes with the advent of machine learning algorithms as they only become better at making decisions with increasing data, being more than competent at analysing it all.
As we are able to collect more data and our AI becomes more capable of analysing it, this will shift the balance in favour of centralisation.