Because French is derivative of Middle Ages (AKA Vulgar) Latin. The main body of base vocabularly is shared with Spanish, Italian, Romanian and other Romance languages and it shares only some vocabulary with old German (of which the Franks spoke). Technically Germanic and Italic languages are very distantly related members of the same Indo-European family but it's such a distant relation that it's not relevant when comparing the languages today on an informal level.

Similarly you could ask why Japanese isn't more similar/closely related to Korean considering the Yamato people of Japan have a large portion of Korean genetics and it comes down to the fact that ethnic groups do not always retain their language. Japanese and Korean are often theorized to be related, but it is very a matter of debate - as /u/keyilan noted and corrected me below. While you can look at Japanese and Ryukyuan languages like Okinawan and see how much closer they are in a number of ways.

Check out Walter & Walter Dictionnaire des mots d’origine étrangère, Paris, 1998 if you've more interest in the subject.