Two major factors preserved the Romance dialects of Gaul: a population advantage and a relatively limited area of Frankish settlement.

Demographic advantage of the Gallo-Romans

Estimates of the population of Gaul vary, with some historians (i.e. Beloch) giving a total of 3.0-4.5 million people, whereas others (i.e. Ferdière) far exceed this with estimated ranging between 10.0 and 12.0 million inhabitants. The later figure is closer to the majority of the modern estimates, though both concern Imperial times, not the Migration Period. In any case, even if the population of Post-Roman Gaul was halved or even quartered for the sake of argument, the Gallo-Roman population vastly outnumbered the Frankish Confederation, who, in the 4th and 5th century, could field a maximum between 10.000 to 20.000 tribal soldiers and probably constituted a total no greater than 150.000 to 200.000 people.

Numerical superiority alone does not guarantee retention of native languages though, however it can be an important factor especially when, as in the case of the Franks, when the society of the political and military elite has little to no technological advantage of the subject society, as this tends to lessen the need for excessive word loaning, which in turn can be one of the factors leading to language shift.

Instead, the higher demographics of Romance-speakers in Gaul and their on par or slightly elevated technological level provided incentives for some of the Frankish elites to become bilingual and, eventually, monolingually French.

Note: The figures given above for the Romans are more accurate, being based on archaeology, Roman records and demographic projections linked to agricultural land use; whereas those of the Franks and other Germanic tribes are based mainly on regressively calculating approximations of military manpower fielded by them. Nevertheless, the basic tenant of any comparison between the two will always hold: the Franks were far exceeded in number by the Gallo-Romans.

Limited settlement

The Frankish conquest of Gaul, doesn't equal the Frankish settlement of Gaul.
When Clovis (466-511) expanded the Frankish Kingdom to include much of modern Northern and Western France, he did so from a territory behind the Roman limes in which his people had lived for over a century, having being settled there by the Emperor Julian in 358 CE as a foederati following their defeat by the Romans. This area roughly corresponds to the southern Netherlands and Northern Belgium. It was in the territories directly adjacent to these Frankish heartlands that the Franks actually settled in numbers high enough to impact the linguistic landscape. This toponymy map can give some impression of the extent of actual Frankish settlement (signified by the suffixes -ghem, -hem) in France proper.

The Frankish language today lives on in parts of the above areas, only it's called Dutch. Dutch is the direct descendant and closest extant language to Frankish language. In France, Dutch used to be spoken in the area around Calais until well into the 18th century. After the French Revolution however, the French government started a process of francization which affected all minority languages of France, not just Dutch, but which began their decline. Today, only a small minority of about 20.000 to 50.000 people in the Northwestern most corner of France still speak Dutch.

Elsewhere, the Franks followed the basic pattern of most if not all of the Germanic kingdoms of the immediate Post-Roman Period, in which the Germanic-speakers were a clear minority and were (like the Vandals, the Visigoths, Ostrogoths, Lombards, Suebi and the Burgundians) eventually assimilated by the surrounding population around the end of the 9th century.

Epilogue

By the time of Charlemagne, the term "Frank" as a more or less specific ethnic marker had become meaningless. Francia now encompassed nearly all of France, the Low Countries, Switzerland, Northern Spain, Northern Italy and most of Germany and Frank could be used for any inhabitant of this realm, regardless whether they spoke a Romance language, a Germanic dialect deriving from Frankish proper or an unrelated form of Germanic. With the formation of the Holy Roman Empire, West France (formerly merely one of three existing Francia's) now effectively became Francia proper and its population (by and large speakers of dialects of, or closely related to, French) became the "Franks", i.e. the French.