I grew because it was efficient (compared to competitors) and because it was fuelled by the bounty of conquests.
It fell because its success attracted oligarchs - and, historically, once the oligarchs get control, progress goes into reverse, the society collapses and 25% of the population die.
It's all explained the the book "Progress Vs Parasites".
The Swiss have the right idea: direct democracy. Oligarchs are steadily increasing their control over the rest of Europe.