What were the incentives in the past and what are they now?

Are you completely ignoring cable news networks and their decades-old rating mechanisms. Mass-media polarization started decades ago, what the internet made possible was to target the long tail of niches. 40 years ago, a conspiracy theory targeting Jordanian bakers[1] would have fizzled out after the first print-run of flyers in a small town, but today, it can be a self-sustaining forum with members all around the world egging each other on in their echo chamber.

1. As a random example

Even mass media wasn't the beginning of it.

There's lots of examples to look at in history of "outrage" that spread through a huge part of the world, for example inquisitions or witch trials.

Slower, but I'm sure if you thought harder or had better knowledge of history than me you could come up with examples of things that happened before even the printing press was invented.

There's probably plenty of examples in Roman history that spread out from Rome to its conquests.