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The main reason was that the allies were prepared to fight WW1 all over again. The Germans had very different ideas. The allies were ready to fight a static trench war in Belgium. Problem was that Belgium had declared neutrality. Allied officers were not allowed to coordinate with Belgium before hostilities broke out. Then they discovered that the positions they were supposed to occupy weren't ready. Even worse: the Germans also violated Dutch neutrality, which caused the allies to (try) to set up defenses - for which they weren't prepared - much further north into Holland. French troops were moved to Breda (North Brabant) but never even got there.

• https://en.wikipedia.org/wiki/Mechelen_incident
• https://en.wikipedia.org/wiki/Manstein_Plan
• https://en.wikipedia.org/wiki/French_war_planning_1920%E2%80%931940
• https://en.wikipedia.org/wiki/Maurice_Gamelin
• https://www.google.co.th/maps/place/Ch%C3%A2teau+of+Vincennes/@48.8425682,2.4004746,13z/data=!4m5!3m4!1s0x47e672bce3f88169:0xdd022654373467c7!8m2!3d48.8425682!4d2.4354935?dcr=0
• https://en.wikipedia.org/wiki/Franco-Belgian_Accord_of_1920
• https://en.wikipedia.org/wiki/Panzer_I
• https://en.wikipedia.org/wiki/Panzer_II
• https://en.wikipedia.org/wiki/John_Vereker,_6th_Viscount_Gort
• https://en.wikipedia.org/wiki/Fort_Eben-Emael
• https://en.wikipedia.org/wiki/History_and_culture_of_substituted_amphetamines#Military_use
• https://www.youtube.com/watch?v=Q95__gXJ9l0

The main reason was that the allies were prepared to fight WW1 all over again. The Germans had very different ideas. The allies were ready to fight a static trench war in Belgium. Problem was that Belgium declared neutrality in 1936.

That created huge problems for the allied planners. Allied officers were not allowed to coordinate with Belgium before hostilities broke out. Then they discovered that the positions they were supposed to occupy weren't ready. 

Even worse: the Germans also violated Dutch neutrality, which caused the allies to (try) to set up defenses - for which they weren't prepared - much further north into Holland. French troops were moved to Breda (North Brabant) but never even got there.

• https://en.wikipedia.org/wiki/Mechelen_incident
• https://en.wikipedia.org/wiki/Belgium_in_World_War_II#Neutrality
• https://en.wikipedia.org/wiki/Manstein_Plan
• https://en.wikipedia.org/wiki/French_war_planning_1920%E2%80%931940
• https://en.wikipedia.org/wiki/Maurice_Gamelin
• https://www.google.co.th/maps/place/Ch%C3%A2teau+of+Vincennes/@48.8425682,2.4004746,13z/data=!4m5!3m4!1s0x47e672bce3f88169:0xdd022654373467c7!8m2!3d48.8425682!4d2.4354935?dcr=0
• https://en.wikipedia.org/wiki/Franco-Belgian_Accord_of_1920
• https://en.wikipedia.org/wiki/Panzer_I
• https://en.wikipedia.org/wiki/Panzer_II
• https://en.wikipedia.org/wiki/John_Vereker,_6th_Viscount_Gort
• https://en.wikipedia.org/wiki/Fort_Eben-Emael
• https://en.wikipedia.org/wiki/History_and_culture_of_substituted_amphetamines#Military_use
• https://www.youtube.com/watch?v=Q95__gXJ9l0

• https://en.wikipedia.org/wiki/Mechelen_incident
• https://en.wikipedia.org/wiki/Manstein_Plan
• https://en.wikipedia.org/wiki/French_war_planning_1920%E2%80%931940
• https://en.wikipedia.org/wiki/Maurice_Gamelin
• https://www.google.co.th/maps/place/Ch%C3%A2teau+of+Vincennes/@48.8425682,2.4004746,13z/data=!4m5!3m4!1s0x47e672bce3f88169:0xdd022654373467c7!8m2!3d48.8425682!4d2.4354935?dcr=0
• https://en.wikipedia.org/wiki/Franco-Belgian_Accord_of_1920
• https://en.wikipedia.org/wiki/Panzer_I
• https://en.wikipedia.org/wiki/Panzer_II
• https://en.wikipedia.org/wiki/John_Vereker,_6th_Viscount_Gort
• https://en.wikipedia.org/wiki/Fort_Eben-Emael
• https://en.wikipedia.org/wiki/History_and_culture_of_substituted_amphetamines#Military_use

The Germans weren't surprised that their plan did work, they were far more surprised it worked so well. For example, Von Rundstedt's infamous Halt order at Dunkirk showed the German high command got cold feet. He was sure the allies had prepared a counter offensive or a big trap.

• https://en.wikipedia.org/wiki/Mechelen_incident
• https://en.wikipedia.org/wiki/Manstein_Plan
• https://en.wikipedia.org/wiki/French_war_planning_1920%E2%80%931940
• https://en.wikipedia.org/wiki/Maurice_Gamelin
• https://www.google.co.th/maps/place/Ch%C3%A2teau+of+Vincennes/@48.8425682,2.4004746,13z/data=!4m5!3m4!1s0x47e672bce3f88169:0xdd022654373467c7!8m2!3d48.8425682!4d2.4354935?dcr=0
• https://en.wikipedia.org/wiki/Franco-Belgian_Accord_of_1920
• https://en.wikipedia.org/wiki/Panzer_I
• https://en.wikipedia.org/wiki/Panzer_II
• https://en.wikipedia.org/wiki/John_Vereker,_6th_Viscount_Gort
• https://en.wikipedia.org/wiki/Fort_Eben-Emael
• https://en.wikipedia.org/wiki/History_and_culture_of_substituted_amphetamines#Military_use
• https://www.youtube.com/watch?v=Q95__gXJ9l0

The Germans did use a lot of amphetamines to get the most out of their troops, but with mixed results. Yes, they could fight much longer. But once the stuff stopped working, you had to rest. There and then. And very long, several days at least to recover. It was very useful on limited targets, such as the capture of Fort Eben Emael. Giving it like Smarties to advancing infantry units wasn't a good idea. Fortunately (for the Germans) the French high command was bad enough so it didn't matter much.

The Germans did use a lot of amphetamines to get the most out of their troops, but with mixed results. Yes, they could fight much longer. But once the stuff stopped working, you had to rest. There and then. And very long, several days at least to recover. It was very useful on limited targets, such as the capture of Fort Eben Emael. Giving it like Oreos to advancing infantry units wasn't a good idea. Fortunately (for the Germans) the French high command was bad enough so it didn't matter much.