[choeger]

I think the most plausible explanation is the lack of slaves that stems from the steady downfall of the Roman empire. When Germanic warlords were pretty much forcing their way into the empire they actually accepted the life style of Romans. But a few hundred years later they switched to feudalism. Why, though?

I think that the Roman empire was founded on a source of cheap labor, slaves. It allowed for massively productive agrarian complexes and thus fueled trade and literacy. But when the empire got into trouble, the price of slaves must have risen massively. The consequence must have been famines and less labor for anything else than crops, in particular less professional soldiers.

[coliveira]

You can only maintain slaves if you have a powerful army capable of defeating other civilizations and maintaining them captive. That's how ancient empires did it, it is also how Europeans and Americans did it in the new world. During early middle ages each location had armies that were good enough only to barely keep their own survival. So, slavery was out of question as a practical way to get labor. They had to organize their own society into social classes to guarantee that someone would do real work. Instead, Romans could just start another war or trade the already existing slave population.