Even though the idea of human equal rights has a rather long history, it wasn't a reality up until the late 18th century. Nobles still had more or less absolute power over the lower classes. The lower your caste was, the harder it was to enjoy a fair and just legal system.
Throughout history there have been several rebellions where the common man tries to stand up against his oppressors but it has always been quelled and things have gone back to status quo. Perhaps a king or two was replaced but the system as it was remained pretty much intact - nobles at the top, commoners at the bottom.
The French Revolution marked a turning point. Not only were the rulers overthrown for real but this time the revolutionaries managed to keep it that way. And although the revolution sparked a few years of terror, it planted a seed of equal rights that would spread to the rest of Europe and large part of the world.
Several other countries, like Sweden imposed more equality for the common man around the same time. The king of Sweden, Gustav III (1746-1792), created several laws that gave commoners more rights and nobility less so. He also abolished the death penalty and torture for all but the most severe crimes and he allowed Jews and Muslims to practice their religion openly. All in all, a rather humane ruler at the time.
So why did all this happen at this particular time in human history? There must have been several times where the revolutionaries were fed up with their rulers the same way as the French were. Was society simply not ready for equal rights? Or is this one of those questions that is really not possible to answer?