The simple answer is that France in our modern conception of it didn't exist in 1066, and doesn't even begin to exist until the late thirteenth century.

The more complicated answer is that when William invaded England, he became King of England. In that respect, he had no feudal obligation to the King of France, because the King of England had no such tie. However, he was still Duke of Normandy, and the Duke of Normandy was the vassal of the King of France. Thus, William and his descendants claimed sovereignty from the higher title of king, while the kings of France claimed overlordship because of the original oath of vassalage. This is precisely the source of English-French conflict, not a preventative, a situation that only became worse after the kings of England claimed the duchy of Aquitaine after Henry II married Elanor of Aquitaine in 1154.