[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.