What were the reasons for the establishment of feudal fragmentation in the history of Europe?

In Western Europe, there were several reasons for feudal fragmentation. Charlemagne’s sons were unable to retain power after his death. There were weak trade ties between the lands, because the economy was mainly subsistence, there was no need to organize trade. Feudal estates supported themselves independently. Gradually they became, in fact, separate states, had their own way of life, their own orders. This led to feudal fragmentation.