The Ardennes were almost completely undefended, in both men and fortifications, because the general idea is that terrain was too harsh to allow any effective attack in that direction. That proved to be untrue: in hindsight, the Ardennes should have been either as fortified as the Maginot line, or as defended as the rest of the Belgian border.
That was the exact idea. Most of the Franco-British army was focused on that border, waiting for exactly that move: the Maginot Line was meant to channel the German attack exactly where everyone was expecting it. If the Germans go through central Belgium, all the plans the French and British mounted up are now validated and in effect. And, while the Germans can probably push them back more than they would expect, there's no horribly blind spot like the Ardennes were.