The Ardennes is a bit of surprisingly rough country. Elevation varies by 1,000 feet in places. It was lightly populate, heavily forested, with few roads. It has boggy swamps and swift mountain rivers famous for their little waterfalls, most especially the waterfall of Coos. That's great for outdoor enthusiasts and tourists, not so great for military logistics.

The French did maintain a continuous front. So there were troops facing the Ardennes. But the French didn't have enough tier one troops to man the entire front. So they took a look around. Alsace and Loraine was the most direct route into France, and the one used in 1870. So that's where they placed fortifications and good troops. Northern Belgium was less direct, and was a neutral country. But it was also mostly flat and mostly clear farmland, making it an attractive attack route, and the one used in 1914. So excellent troops were placed to block the Germans there. That meant that the less great troops were placed blocking the Ardennes.

In fact the French had very successfully predicted the original German plan. The first draft of Fall Gleb (Case Yellow) called for exactly the large movement through Belgium that the French were expecting.

Manstein's plan to attack through the Ardennes was counter to traditional military logic and was strongly opposed by a lot of the German command. And it should be pointed out that a lot of the problems predicted by the plans detractors did come true. Logistics were a nightmare. There were weeks long traffic jams. The units did at times run out of ammo and/or fuel. But in the end the advantage of attacking a weakly defended point outweighed the disadvantage of attacking through terrible terrain.

In the end the French mistake wasn't that they left the Ardennes lightly defended. It was probably the right place to put their second tier troops. The mistake was that they should have had a more robust reserve force, recognized the developing attack, and committed the reserve force more decisively.