I know they say hindsight is 20/20, but it seems like they just went "well surely they won't do that again" for arbitrary reasons. If not, was there a specific reason/reasons that they ignored that possibility?
Quite to the contrary, the western Allies did expect Germany to attack through Belgium again. They planned to use most of their forces to meet the german advance in Belgium, while using the Maginot Line for static defence on their right flank. The Ardennes were considered to be impossible to penetrate for armoured forces - which was the real mistake.
There are many myths surrounding the Maginot line. Let me state some facts.
The Maginot line cost about 2% of the French army budget 1930-1936.
The Maginot line was intended as a force mulitplier. The Germans had a distict advantage in population on the French, and thus a larger manpower pool. The French knew they could not match the Germans man for man all over the front. The Maginot line was intended to free up the French mobile forces to fight in the west.
The ground in northern France is mostly clay with lots of water - it is very hard to build heavy fortifications in these conditions.
The French, knowing that the Germans had an advantage in manpower wanted to make sure the Belgians and their 650 000 men army was on their side. Shutting them out was thus politically very hard.
There's substanital parts of the French heavy industry and coal mines just behind the border - the French had suffered quite a bit from having this area under German occupation or the frontline going through them in ww1. They thus wanted the front to be further away than heavy artillery range from these vital areas - which meant it had to be in Belgium, and not in France.