Hello,

I've read a lot about this period so I can try to answer this question. First of all, it was absolutely not due to the legacy of the Roman administration. The Roman empire had, of course, a profound effect on the Franks, but mostly in the spheres of language, culture, writing and obviously the foundation of many Frankish cities by the Romans. The administrative of the empire collapsed in the West during the 5th century. Frankish government was largely "supervisory" -- that is to say, Charlemagne did not go around managing day-to-do affairs or legislating about small issues -- 8th-9th c. government was nothing like what we would think of as government, and the state existed largely in the minds of the elite rather than in an institutions. The king would appoint counts and later missis dominici around the kingdom, who were usually local aristocrats. Therefore, aristocratic and royal power worked together, and someone could come to hold more local power by being appointed to a royal position. Royal frankish government relatively was more intense in the heartlands of the kingdom between the Loire and the Rhine, were the preponderance of royal estates lay. Royal estates were actually controlled by royal officials in a more direct sense, and served as the basis of royal wealth and government, where the king could distribute royal largess in the form of beneficed estates and royal offices to local noblemen.

As for why the Franks were so successful, it is due to a range of reasons, many coming down to individual factors. Had not Clovis, Charles Martel or Charlemagne been such effective war leaders the empire might have collapsed (as it almost did/had done before Charles Martel laid the groundwork for the Carolingian empire) or never come to be. Clovis' conquests and embrace of Christianity proved allowed for the establishment of Frankish rule in Gaul and parts of Germania. Charles Martel was able to unify the Merovingian kingdoms and leave his son Pepin in a position to claim to crown itself (before 751 the Carolingians were mayors of the palace rather than kings). Pepin the Short and Charlemagne were both effective leaders and held the empire together. The Carolingians reestablished royal authority by creating the above mentioned counts and missi dominici, conquering recalcitrant, breakaway and then new territories. Royal authority proceeded to collapse yet again during the 9th and 10th centuries, only to be rebuilt in later centuries.

Hope this helps, let me know if you have any more questions or I haven't been clear. For more reading, I would suggest "The Carolingian World" by Marios Costambeys, Matthew Innes and Simon MacLean, which is one of my favorite books on the period, as well as Rosamond McKitterick's work, which I find a bit dry but is nonetheless very important to scholarship on the period.

Can I just get some clarification as to what time period you're referring to? Are you asking about the transition from Roman Gaul to the [Merovingian] Frankish kingdoms (which happened in the late 5th and early 6th centuries) or the transformation of the Merovingian kingdoms into the Carolingian empire (mostly taking place in the late 8th century)? They're two pretty different situations, although I don't think either involved any major change in technology or administrative knowledge (in the modern sense anyway).