A bit of a tangent, but the more complete reason depends on what civilization (Aztecs/Incas), the common factor is an extreme loss of life due to old world disease.

Additionally, for the projectile velocity at the time the gambeson-like garment the Aztecs had available was surprisingly effective.

I share your concerns, but also I don't think this analogy is very close. Historically the reason why commoners could be kept oppressed is because the relative amount of "firepower" available to individuals was fairly small to begin with, and easy to regulate. Many places banned military weapons like swords for the commoners, for example, precisely so that they couldn't quickly form a militia capable of challenging their feudal lord's retinue. I don't think that's possible in the modern world, though, because even with heavy regulation of arms, the stuff that's readily available (or can be put together from things that are readily available) is already too destructive to contain.

The modern rulers rely more on brainwashing and less on direct oppression for this exact reason. Not that the latter doesn't happen, mind you, but I also can't think of any modern day regime that is sustained solely by force, without some measure of popular support.