Some of the suggested causes of the Allied defeat were:
* Treason: this theory was very popular at the time of events. A Fifth column was supposed to be cooperating with a host of disguised German agents. After the war this was conclusively shown to have been a case of mass hysteria, but such stories are still repeated in some popular accounts.
* Equipment imbalances. Contrary to popular opinion, often assuming that the Germans had a completely mechanized army, while the French only had WWI equipment, there was no general disparity of armament levels. It's generally acknowledged that in a pure battle of attrition, the Germans couldn't have won.
* Communication difficulties. The French communication system relied almost entirely on the public telephone network rather than radio. The telephone lines were often cut, either by military action or by acts of sabotage and often the only way of sending messages to the front was by dispatch rider. Allied Commanders complained that they often had no information for days and when it did arrive, it was hopelessly out of date.
* Defensive attitude: French overreliance on the Maginot Line, a chain of forts built along most of the French-German border. It is undisputed that the French left the strategic initiative to the Germans.
* Failing strategy: General Gamelin's decision to send his best forces north to defend against invasion through the Low Countries, combined with Hitler's decision, against the advice of the German General Staff, to adopt the Manstein plan after an aircraft that was carrying a copy of the original invasion plan crashed outside German territory.
* Outdated tactics. It's often assumed that there was a neglect of tank warfare by the French. The French built a larger number of modern tanks than the Germans did and these were on average better armed and armoured. Also it isn't true that they divided them among the infantry in penny-packets: even the independent tank battalions were combined in Groupements and allocated at army level. However, the French suffered from an inflexible division in infantry and cavalry tanks: ironically the former were poorly trained to cooperate with the infantry and so couldn't execute modern combined arms tactics. In theory the operational doctrine of both armies was based on partly mechanized manoeuvre warfare; in practice the French shied away from it, while the best German field commanders were so bold as to let it develop into pure Blitzkrieg if the situation allowed.
* Quality and guidance of German troops in combat. The French population was much smaller and more aged: they had to draft a lot of elder men to form so-called "B"- divisions, which they then couldn't train properly as most professional instructors were needed to man the "A"-divisions. To compensate for the lack of capability, French infantry doctrine stressed the importance of methodical procedure, leading to inflexibility. The Germans too had many insufficiently trained reserve divisions; but those infantry units used for the breakthrough all consisted of young and well-trained men. Their officers on the tactical and operational level were considered the best in the world.
* More controversially, defeatism (or a lack of willingness to fight) among the French and particularly French leaders. This hypothesis was very popular in France itself with such books as Strange Defeat by Marc Bloch. American journalists, being neutrals at the time, observed much of this on both sides: the German populace wasn't enthusiastic about the war either. Most German generals were opposed to the campaign.
* Intense French losses during World War I caused an inability for the French to regenerate the resources necessary to defend France in 1940.