France wanted to weaken Germany after World War One, and annexing the Rhineland was one of the options they suggested. Germany had more people than France and that was considered to be a threat to the future of France. The other allies said no to annexing the Rhineland.
Some French - Foch notably. There were a good many voices in France who rejected the idea. One of the problems is that to claim the Rhineland, the French would need to violate the logic used to press their claim to Alsace Lorraine.
And all this applied after WW2 as well. And after WW2, the need for a military frontier on the Rhine is lessened since the Allies properly finished the job that time around and Germany was crushed and occupied.
What may be possible is for France to reject merging their zone of occupation into the Anglo-American Bizonia and also separate it from the Soviet zone, meaning Germany splits into 3 - Bizonia, East Germany and... West Germany?
It would require much better Franco-Soviet relations and much less threatening Soviet behavior.
fasquardon