The main reasons why France lost so quickly in World War II was that it planned poorly for the war while the Germans planned effectively.
After World War I, France wanted to ensure that it would be immune from German attack. The main thing that it did to accomplish this was to build the Maginot Line. This was a massive series of defensive fortifications between France and Germany. The line was meant to prevent a direct invasion from Germany. When war started, the French relied on the line to hold the south and sent troops into Belgium to prevent a German invasion from that direction.
The Germans, however, fooled the French. The Germans attacked through the Ardennes, which the French thought were too rugged and forested to penetrate. This allowed them to flank and encircle Allied troops in Belgium. It also got them in behind the Maginot Line.
In addition to this, the Germans had better tactics. They had been planning for offensive war and had prepared a doctrine of blitzkrieg with highly mobile troops being closely coordinated with air, armor, and artillery. The French, by contrast, had been planning on a defensive war and did not have any innovative doctrines that could be used to beat back the Germans. For these two main reasons, the French lost quickly to the Germans.