Why was the life of the peasants in the early Middle Ages very difficult?

After the fall of the Western Roman Empire, the Dark Ages began in Europe. On the territory of the former empire, barbarian states were formed. The main goal of the barbarian kings was unlimited wars with their own kind. The peasants were just rebuilding their villages after the barbarian invasions, when more and more raids took place here, after which they could take people into slavery. In addition, during this period, Christianity is spreading throughout Europe. At that time, none of the peasants knew how to read and write. The priests understood that it was easy to manage the illiterate population, so they used it in every possible way.