Back in the day, when Nordic people honored Odin and Thor and Freya, and Celts prayed to Lugh and Brìd and their peeps, (the Tuatha Dé Danann) there wasn't just one king in a country. And also, countries weren't quite so firmly defined. Really more like territories; strongly held in the center filtering out to disputed areas on the periphery. By the way, we're really only talking about a few hundred years into the Common Era here - not back in the Bronze Age.
As Europe was gradually infected with converted to Christianity, things began to change. Instead of one king for something like a bioregion in federation with other similarly-powerful monarchs, we start to see a vertical relationship. Bioregional kings start to report to a national (sort-of) high king.
The squeeze is starting to happen.
In the monotheistic cosmology, there is a finite amount of power available, and the more people who are sharing it it, the less powerful each is. Just like Yahweh demanding supremacy among the Semetic gods, we start to see high kings demanding supreme power over their formerly sovereign sub-sovereigns.
|Note to Self|
|I need to get some good citations going for this particular point, since I don't expect anyone to just believe me. I'll get right on that.|
Pretty soon after that, none of the local thugs nobility are called kings anymore. They've all been demoted to lower ranks, just like when Yahweh handed the entire Cananite pantheon its collective hat in one swell foop.
So, how did this thirst for absolute power find its way so far West? Is it just a natural step in the evolution of society? Not in my opinion. It's the thick edge of the monotheistic wedge. If your whole cosmology is based around the idea that there's just one sovereign power in the whole universe, which direction to you think society is going to go in? Pluralism? Democracy? Ummm. No.