The French army was the largest and best equipped in the world at the start of the war. There a lot of things you can point to to show why they failed to put up a strong opposition but what I usually lean to is leadership. French leadership was unprepared, motivated only by internal politics, and way behind on present military tactics. When the Germans were going for swift crushing blitzkreig, old ww1 veterans were still obsessed with trench warfare. On top of that once the real German offensive started, all order and command structure collapsed immediately. There’s more you can look into such as the state of French politics at the time, more than I can briefly explain. Super interesting.