As pointed out, there was no need for the line after Germany had been conquered. Also, modern warfare meant that these fixed fortifications were not particularly effective given their cost.

Its also not as simple as Germany just walking around the line. The line stopped at the Belgian border for good reason. Belgium was nominally an ally, and building a wall behind them would have indicated to them that they would be abandoned in the face of German aggression. French military doctrine also favoured attack over defence and they had no intent to shelter behind a wall, but rather intended to take the fight to the enemy. The wall was not so much intended to stop the Germans, as to funnel them north through Belgium where the French army could advance with their right flank hinging on the wall and engage the Germans as far forward as possible. This was designed as an counter to what the Germans did in the first war.

What the Germans did do is attack the weekly defender right flank of the French line through the Ardennes forest. This sector was believed to be largely impassable for an army and was lightly defended. It was a massive gamble for the Germans and it worked. They broke through here and got in behind the advancing French army, cutting it's lines of communication. Was pretty much game over for the French from that point on.