Only a few thousand books from varying opinions/perspectives have been written on this subject. All more or less state the same things, some authors trying to pinpoint what they feel was the crucial fault in French pre-war planning, while others blame a myriad of different problems. The truth is it's all over with some problems having more or less importance, relative to other problems.
Pre-war France was wracked with political and civil strife, especially into the Depression era of the 1930s, with extremist politcal forces vying for power, at times violently so. In general terms, one could find the far Left was pitted against far Right, socialists vs. communists, upper class industrialists vs. powerful labor unions, trendy artists mixing it up with trendy Trotskyists against the religious Capitalists, and throw in a few anarchists just for added fun. Governments came and went with dizzying speed that made even the Italians jealous. All this and the fact that by some miracle the country didn't fall into civil war, it is quite amazing.
By the time war was declared, there were indeed many who thought this would get the people together and forget about old political and social issues/rivalries. To some extent this was true, in other ways it didn't happen. The Non-Aggression pact between Germany and Russia pretty much gave the green light to the communist factions to continue their shenanigans, much of it in the factories and defense works. There were several well recorded examples of sabotage. The defense industries also share in the blame, as a streamlined mass-production plan was never invisioned until it was too late. Most planes were still being assembled one rivet at a time, and on one plane at a time. Great for quality purposes, bad for quick production. Also, there were plenty of lobbyists pressuring the government for fulfilling contract payments on old and obsolecent designs while more modern designs sat on the back burner.
The new draftee in the army isn't going to fight so hard for a system which he may feel is so screwed up and unjust to his liking.
Militarily, as some already pointed out there were certainly too many old minds clouding the halls of St. Cyr for any younger and more imaginative students to get through. A very few, such as then Col. de Gaulle had different ideas and even wrote books detailing his thoughts. "The Edge of the Sword" (1931), and "Towards a Professional Army" (1933-34) are both decent reads, explaining the need to adopt more mobile forces of tanks and infantry, and the development of a core of professional soldiers not reliant on draftees. Most of the military heirarchy viewed these books as a threat to their long established belief of strong fortifications. Afterall, they couldn't have someone telling everyone else that the Maginot Line was a bad idea, after having dumped all that money into it. The political establishment viewed anything resembling an army of professional soldiers as a threat as well, professional armies historically being the tool for coup d'etats by the power hungry .
The idea of the Maginot line itself, while always ridiculed by those who don't understand the mindset behind it, is not all that silly. Interlocking forts with a demi-brigade worth of interval troops in between (sometimes more), with hundreds of artillery pieces and machine guns was a very strong deterrent indeed. The biggest flaw in the idea was not so much it's design, but the fact that it relied heavily on completion of it's northern sector in Belgium. This would connect it with Eben-Emael. Belgian neutrality, and thus the incomplete fortified line meant the entire northern part of France, from the Ardennes to the Channel was exposed. A secondary flaw, revealed immediately afterwards, was there there was no "plan B" in the event something like this would happen.
As for military operations, I'm more convinced that poor communcations was perhaps an overriding cause for the defeat. Units not recieving their orders until it was too late, if they recieved them at all. Many times they recieved orders to move to areas already under enemy control. The over reliance of messenger courriers on motorcycles, many of which were caught in accidents and traffic jams was another symptom of this communication problem. Commanders who placed themselves far from the action led to indecision and since most French troops looked back on their training to follow orders and not innitiative meant many of them would be sitting around waiting for a senior officer, ANY senior officer to tell them something.
On par with the lack of communication, and related to it, was the nearly complete lack of coordination between the army and the airforce, ensuring that mission support would be sporadic at most. Between the Allies, the lack of a supreme commander to handle all forces didn't help much at all, and this made joint operations anywhere from very difficult to non-existent. During the darkest hour, General Bilotte was chosen and approved by the Brits and French, but he was killed in an auto accident very shortly afterwards, which was probably the best thing that could have happened to the poor man, as bad as that may be.
The troops themselves ranged from excellent and motivated to poorly trained and indifferent. Mobilization was very quick and very good. Infact it was so good that men had to be sent back home as the defense industries were affected by all of their workers being called up for duty. The difference in "Class A" troops and "Class B" troops was already known and there seems to have been little effort to rectify the situation. There was a good deal of distrust among some troops towards their officers in these "B" units. These units were also known to have their fair share of social and political misfits. Those "Class B" troops would unfortunately be in the crucial areas around Sedan where the Germans would breakthrough. It wasn't by some fluke the Germans hit here, they knew this was a weak point for this reason, and also that the best and the worst of the French armies was hinged at this particular location.
A variety of other problems could be listed, but the above are what I believe to have been the worst cases. As I've posted before, and excellent and very fair and objective account on pre-war France - political, social, and military considerations - is Julian Jackson's "The Fall of France". I think one of the reason why this has recieved so much coverage among historians, is that this represented a perfect storm of things that you hope do not happen, but they do, and you have no way of knowing it until it's too late. It's a situation that could have happened (and could still happen) to any country, under the right circumstances.