Why are many barbarian kingdoms short-lived? What caused the long existence of the Frankish kingdom?

After the fall of the Western Roman Empire, many states were formed on its ruins. These states formed tribes that came from the banks of the Rhine and from other barbarian regions. Many barbarian kingdoms were short-lived because of their desire to destroy everything Roman that still remained in the occupied lands. With the peoples who lived in such territories before the fall of the Roman Empire, the barbarians did not try to build good-neighborly relations, immediately placing themselves in the position of masters.
The state of the Franks is fundamentally different in this respect. This tribe was more civilized than the rest. Having conquered the fertile and developed Gaul, the Franks began to actively cooperate with the remaining Roman administration, adopt laws and everything useful that the empire could give. The Franks initially assumed the role of defenders of the empire, not its destroyers, which earned them the love and support of the indigenous population.