The best book for explaining this is Alistair Horne's To Lose a Battle. I don't have the book here with me (it is literally beside my bed, as I am re-reading it this week) but I will try to explain a bit.

Distilled down to its simplest, the answer is that victors tend to prepare for the last war, losers prepare for the next. France's army was large and strong. It was though, deceived and outclassed.

For example, the French underestimated the speed of the German advance. During the Great War, armies could not move faster than their artillery could advance. The introduction of the Stuka dive bombers, and their use in the close-support artillery role allowed the German advance to move a lot faster than France could react.

In the Great War, France's biggest battle was fought at Verdun, where the lines barely moved for months. Hundreds of thousands of lives were spent fighting over a handful of forts. Interwar France therefore built enormous networks of forts, with underground railways and barracks, all along the German border. When war came though, it was mobile, fast and avoided these forts entirely. The French army though kept these forts well manned right into June 1940 when they were surrounded.