It's quite simple. The advancing forces were not able to prevent that the defenders re-established an effective defence (defence with little to no further loss of terrain in the short term) shortly after the old defence was broken.

The Germans penetrated at Sedan, broke through tough reserves on some elevated terrain a few kilometres later (Stonne etc) and then didn't allow the French to re-establish a new defensive line until hundreds of kilometres farther west.
It was like a boxer who forces his opponent to stumble backwards for 10, 20 metres, not allowing him to regain his balance and stiffer defensive posture earlier. The German culminating point of attack was quite far.

On the defence in 1944 they merely made a step or two backwards before they regained their balance, all the while their opponent was technically fast enough to run circles around them (keep in mind the German troops did not only lack motorisation and fuel; they were also largely unable to march in daylight due to allied air supremacy!).


Small unit performance was certainly an enabler, but the real fault laid on the operational level, the grand tactics about the employment of divisions and battlegroups / combat teams. The British were way too timid and the Americans didn't exactly exploit the full potential of their internal combustion engines either.