My reading is that the French wanted to fight purely on the defensive so as to cost the Germans high casualties, and in Belgium instead of France. This dictated their strategic thinking, find a defensible position and hold it, in Belgium. As you know, the initial German plan was the Schlieffen Plan 2 basically, but they feared the plans were captured, and Manstein, a junior general, offered this new plan.
The French also had nothing equivalent to an armored division, their tanks were more spread out and diluted, so they couldn't pose any offensive threat adequately. The one British counterattack gave Rommel quite the scare, but was insufficient. The French just couldn't think offensively at all, it seems to me.