[runako]

Not a historian, but some possibilities:

- some governments were explicitly modeled on the US system

- others were influenced by the US system as they moved from e.g. monarchies

- most countries have some sort of caste system that established interests want to preserve

[lovecg]

Bicameralism appeared very, very early on. There’s a well known case of a missing pig in 1642’s Boston (with a population of less than 2000 at the time) that finally solidified splitting the assembly into two chambers, and that debate has been going on for a while at the time already https://www.americanantiquarian.org/sites/default/files/proc...