In this case "Breakthrough" is less accurate than "Breakdown". Even before the Americans arrived the Germans had far fewer men covering each mile of front than the French + British. In fact, at that point in the war the Germans only wanted a victory that would push the war into peace talks and give them a strong bargaining position. With the failure of the German's "Last gasp" offensive that hope disappeared.
The reason the Allies were able to effect such a large breakthrough is manifold, but the main reasons are lack of men and lack of food on the part of the Germans. Even before the war Germany wasn't able to produce enough food to feed all its people. When the war started and the British blockade began hunger and starvation became commonplace in Germany as well as the army. An estimated 700,000+ German civilians died as a result of the blockade. In this sense the German's final offensive actually worked against them. When the Germans began overrunning French and British trenches they found copious amounts of food and supplies, exactly the opposite of what German propaganda had been telling them. For the first time in years it became obvious to the average soldier that Germany was losing the war. Thus, when the Americans allowed the Entente to continue their offensives, German resistance crumbled.
It's also worth nothing that the Germans themselves largely blame the tank. One German officer wrote that his men, whose training and sense of duty was enough to keep them fighting against infantry, had no problems surrendering to tanks. While this is true, the larger reality of it was that Germany was simply spent at that point. They didn't have the men, they didn't have the money, and they didn't have the food to continue the war any longer. So rather than a "breakthrough" what you are seeing is the final breakdown of the German military machine across all fronts.