The church had a better idea

From A Troublesome Inheritance by Nicholas Wade. Page 144.

A central part in the development of European institutions was played by religion. Religion, Fukuyama argues, was critical first in detribalization and second in instituting the rule of law. The essence of the tribal lineage was the descent of property through the male line. But producing a male heir under medieval conditions of short life expectancy and high infant mortality was far from a sure thing. So the tribes had various strategies for keeping wealth within the lineage. These included cousin marriage, divorce if a woman bore no heir, adoption, and the levirate (marrying of widows to their husband's brother.) In addition, women were not allowed to own property.

The church opposed all these heirship strategies, not because of anything in existing Christian doctrine but because it had a better idea: that people should leave their property to the church instead of their heirs. By the end of the 7th century, a third of the productive land in France belonged to the church. The tribes of Europe, whether Frankish, Anglo-Saxon, Slavic, Norse or Magyar, found that conversion to Christianity soon separated them from their property, robbed them of their influence and paved the way to feudalism.