It was a military decision, made in the heat of the conflict.
It was a prudent one as well. IIRC the bulk of the French VII Army was on Von Rundstedt's thinly stretched left flank.
One must remember that the Allies on paper outnumbered the Germans. The swift victory was surprising to both sides. It is easy to quip in hindsight. However looking at the situational maps without the benefit of hindsight, Von Rundstedt did not make a stupid decision IMHO.
All the best,
Crumpp