France and Germany were both former parts of the Carolingian Empire, but only the latter devolved into a bunch of small states while the former stayed intact. Why?
In answering this I’ll divide my response into two parts for clarity: 1) the area now recognised as more typically “German” had historically had much deeper and ingrained divisions than the French regions, and 2) that nevertheless, the idea that France remained united in the aftermath of the disintegration of the Carolingian Empire is, in fact, oversimplification.
First, much of the German part of the Empire was cobbled together quite late in its history. Places like Bavaria and Saxony were well outside of traditional Frankish lands and were only annexed in the 8th Century or so. This meant that the duchies already in existence there were quite well established into the medieval period, with recognisable family loyalties, lines of inheritance, etc. This in turn stemmed partly from the fact that these German regions were cobbled together from the traditional lands of the various Germanic tribes (of which there were quite a few), usually divided by convenient geographical boundaries such as the Elbe and Saale rivers (see Goldberg’s work Frontier Kingship, Military Ritual, and Early Knighthood at the Court of Louis the German). When the Carolingian Empire was divided by the sons of Louis the Pious, East Francia consisted of these traditional tribal duchies (with some alteration) who still had recognisable leading families and strong bonds of loyalty within them. The state mechanisms of East Francia weakened in the face of these traditional noble families reasserting themselves and providing much of the direction in resistance to outside forces such as the Hungarians. Other “German-ish” areas such as the Low Countries were largely a part of Middle Francia which was riddled with administrative problems from the start. Geographical barriers such as the Alps made communication across the entirety of the realm impractical and it also disintegrated quite readily into the traditional duchies.
Second, West Francia did not continue more or less uninterrupted into the establishment of a recognisable French state. Despite the fact that this area was much more ethnically, culturally and linguistically homogenous than Middle and East Francia, it too was composed of various duchies with their own nobilities, such as Aquitaine, Brittany and Normandy. These dukes definitely tried to assert themselves against each other and vie for power within the developing nucleus of France to the extent that during the early 10th Century the Frankish king actually had fairly little power comparatively and certainly did not command the loyalty of the lands now broadly recognised as France (Simon MacLean’s book on Charles the Fat is the main source here). It didn’t help that the Frankish kings were quite unhelpful in repelling Vikings in this period as people turned to local nobility to assist them. Rather, the French kingdom developed out of the efforts of the Capetian dynasty who slowly but surely managed to secure greater power through marriage, fortunate purchases and straight up conquest (Hallam’s work Capetian France details much of this process). Even after this period, with much of the power in France more centralised, the ancient Frankish practice of granting lands to multiple sons, which would revert to the main kingdom on extinction of the male line meant that France sometimes lost pieces as they became associated duchies (e.g. Burgundy).
Basically, the story isn’t quite as simple as you might think. France was very divided for a long time as well, but Germany certainly did have factors such as geography and historical tribal divisions that made splintering more likely.
I'm not sure you can describe any political entity in the middle ages as a "state". France was, just like Germany, made up of several duckies, counties, arcdioceses, and what not, but they didn't function like anything that resembles a proper state. Both France and Germany, or rather, the Holy Roman Empire were a weird conglomeration of principalities, which paid lip service to a King (France) or Emperor (HRE), but the practical aspects of managing the territories were carried out by the regional rulers, and differed greatly from region to region. Now, HRE was initially rather centralised, from a medieval perspective, but suffered what I would call a breakdown in the late middle ages, in which the Emperor lost a lot of his political influence to the regional magnates. This was codified in the golden bull of 1356, which regulated the elections of the emperors, and guaranteed the indivisibility of the seven electoral principalities, which meant that the electors became the most important Princes in the empire. The electors would of course prefer an emperor who didn't meddle too much in the internal affairs of the principalities, which led to decentralisation. France, on the other hand, was extremely decentralised following the fall of the Carolingians in the late 9th century, which led to a "feudal anarchy" in which the king was nearly powerless in the face of regional princes. However, during the reign of Philip Augustus (1180-1223), the French king amassed vast territories, which enabled him to centralise his kingdom. This, coupled with the rising "nationalism" in france which emerged during the hundred years war (1337-1453) ensured that France came out of the middle ages as on of the most centralised powers in Europe.
Hope this answered your question
For further reading I can recommend: Marc Block "feudal society" George Duby, his study of the macconais region. Susan Reynolds "kingdoms and communities" & "fiefs and vassals" I want to find a few more sources for this, but I am not currently home. I will update this post later with additional sources.