In part the desire to fight on non french terroritory. There was a belfief in a long war. they did not want a long occuptaion and devastation of northern France like in ww1.Couldn't France have changed course in 1936 and extended the ML fortifications to its Belgian border, though? 1936 to 1940 is four years.
They waited until the acutal German invasion.Also, why didn't the Belgians blow up the bridges in the Ardennes after the Nazi invasion of Belgium in real life?