I've got a theory that democracy develops in step with military technology and how "equal" an individual soldier is on the battlefield.
Ancient Greece: Phalanx, requires groups of equal men to stand and fight in formation without breaking. Equipment and training was relatively even. The rich could afford better armor, but that wouldn't turn them into a super soldier.
Medieval: Mounted cavalry or mounted archers. Expensive to maintain a war horse and equipment, years of training required. But a mounted knight was extremely difficult to kill for groups of peasant infantry and the nobility could dominate the battlefield.
Industrial: Once again, large numbers of men wielding cheap firearms with much less training required than horseback combat. America being founded as a democracy probably had a lot to do with recruiting as many militiamen as possible rather than focusing on the interests of a highly trained military elite. One soldier, one vote reflects how much an individual soldier contributes on the battlefield during a time that spanned from musket lines to WW2.
Today: Highly specialized militaries, numbers aren't as important as technology. Based on how democracy has advanced or retreated with the importance of an individual on the battlefield regardless of his wealth or experience, as the scale tips back towards a highly trained military class using expensive weaponry, expect democracy to retreat as the few wield military power over the many.