It’s all down to ideology of race. Within Nazi ideology the French were, if not Aryan, then close enough, while Slavs were a race lower than Aryans, though not as low as Jews, who were slated for extermination while Slavs were meant to be exploited for their labor. The French were a colonial power, and to use a current term, they fit into Nazi views of whiteness. The French could be turned into good Nazis, hence the Vichy regime. Slavs however, were seen as a kind of exploitable labor class meant to be forced to work to serve the Reich.