There's a problem of scale with any centralized authority. While it's relatively easy to have total control over a population of a few thousand, span-of-control problems mean that multiple levels of managers must be involved the larger the populace.
Note that I am not saying that folks are more oppressed now than at any other time in history (aside from obvious examples). I am saying that the ability for a central authority to monitor and control each person's behavior has scaled enormously.
Every large enough organization experiences a tremendous degree of infidelity between the authority's wishes and how those wishes are actually carried out. Most folks look at some huge thing like, say North Korea, and think that there is one person completely in charge, and on paper it looks like that. But reality doesn't work like that at all. Instead central authorities in large systems constantly have to use persuasion, politics, and marketing tactics to get their wishes fulfilled. This was true with ancient Rome and much as today.
So there is a constant give-and-take between the ideals and plans of leaders and the zeitgeist of the ruled. This, along with regularly-changing leadership, allows organizations to learn and adapt. Effective organizations embrace this tension and provide a means for regularly changing course, sometimes in a dramatic way.
But what if leaders didn't have to worry about all those middle-level guys and discontent? What if you could have one central person/group that could monitor each person's actions minute-to-minute, applying various forms of non-lethal pain every time the person steps out of line? Don't like the food prices and want to go downtown to yell at the leaders? Well maybe we'll let you feel like you are on-fire for a few minutes and see how likely you are to try that again. Caesar doesn't have to worry about the Senate or governors in the provinces or the mob any more -- with computers and non-lethal population control, he can effectively manage hundreds of millions as easily as he could some bloke sitting in front of him in the same room.
The state-of-monitoring discussion is for another thread. The key point here is that monitoring and control becomes a lot more palatable if nobody is getting hurt, so by inventing lots of forms of non-lethally controlling folks, leaders can squeeze tighter and tighter. Never before in history has it been that way. When the United States passed prohibition, everybody nodded their heads and drank anyway. Over time, it was painfully obvious what a disaster prohibition was. It's not that way today. Back then, to enforce prohibition you had to have tens of thousands of cops and FBI agents who were true believers and willing to harm and kill people to enforce the law. Those people simply didn't exist. The people who passed prohibition had morals, righteous feelings, and a majority vote on their side, but they wanted the impossible from the population.
Today we're cutting way back on the systemic checks -- it only takes one true-believer in prohibition somewhere in the government to do some data mining of cell phone locations over time to easily find out where all the gin joints are. It only takes one or two guys to deploy some non-lethal force on those establishments to make them off-limits -- all without the nasty political fall-out of people harmed or killed -- or the practical requirement of actually having a big enough percentage of the population that supports enforcement (instead of just enactment)
We live in an incredible time in history. We're seeing this massive system of state control develop on a scale that would boggle the minds of folks just fifty years ago. I'm hoping for a century or two of Pax Romana before we end up with a Caligula -- but I wouldn't bet on it.
Compared to, say, ancient Egypt?