score:4
Despite the Franco-Belgian accord of 1920, the Belgians were skittish about becoming too dependent on, or becoming a protectorate of France. Basically, they wanted a "balanced" relationship with both France and Germany, barely "tilting" toward France.
By 1936, Belgium "thought better" of the above accord, backed off on its provisions, declared its on neutrality, and made its own (leaky) arrangements with Germany.
Upvote:2
There is also another reason. Relations between The Netherlands and Belgium were not exactly friendly after the Belgian revolt until the end of WW2(!). Belgium wanted even to annex Zeeuws Vlaanderen, North Brabant and Limburg after WW1. These tensions led to the founding and rise of the NSB party (Dutch Quislings).
Extending the Magniot Line up to the Dutch border would not be something the Dutch government wanted to see. Given the expense of the fortifications, the political problems north (The Netherlands) and east (Germany) more reasons for France not to extend it.