Why exactly Gaul became the basis for the formation of the state among the Franks?

Since the days of the Roman Empire, Gaul was one of the richest provinces of the empire. Agriculture, comfortable cities, brisk trade attracted barbarians to this province from across the Rhine. As a result, the Franks conquered Gaul and established their state there. Having found a ready-made foundation for building their country, the Franks not only did not destroy, but in every possible way tried to preserve and improve the existing order. This is especially true for administrative entities. Being, in comparison with other barbarian tribes, a fairly civilized people, the Franks preferred not to destroy, but to take the best that was in Roman Gaul.