Why didn't that happen in OTL?
The French army on its own terms was actually pretty good. They inflicted some painful defeats on the Prussians IOTL, with everything going against them. The Prussians just managed to force a fight that played to their strengths and the enemy's weaknesses.
Since I can't see the strategic picture changing, the French need to be better prepared for it. The wars they fought before, though, all didn't really motivate them that way. The problem is, the only options for getting into wars that would do that - wars where they faced strong armies with modern artillery and railway-based logistics - would butterfly away the Franco-Prussian War with near certainty. Maybe if they just paid closer attention to 1866? They could learn something from the Austrian experience.