The American Sherman tank was clearly outmatched by German armor in World War II, so why wasn't it easier for the Germans to hold the line on the Western Front?
I would point to two factors which mitigated the German advantage in armor and allowed the Allies to win on the Western Front in relatively short order after D-Day.
First, there is the issue of fuel. German fuel supplies were running low due to strategic bombing of synthetic fuel plants, among other factors. This greatly reduced the potential effectiveness of German armor.
Second, there is the factor of tactical air support. In the early days of the war, the Germans had air superiority in Europe. By the end of the war, the situation was completely reversed. Armor is, of course, much less effective when the enemy has air superiority as complete as that which was enjoyed by the Allies after the invasion of France.
For these reasons, the German advantage in armor did not translate to an ability to actually hold the line on the Western Front.
One must also note that the Sherman tank like the T-34 on the Eastern front were mass produced tanks, german armor production simply could not compete with Soviet production and U.S. production on the Western Front. German tanks (especially the Tiger and Super tiger) while having superiority in firepower and armor were never produced in numbers to rival that of their competitiors. Also the panzers were held in reserve and thus came into action late after D-day allowing the allies to gain a strong beachhead in Normandy. The terrain of the hedge country also limited the effectiveness of German tanks in the early stages of the Western Front, the above answer about fuel and air support were also key especially in the latter stages of Battle of the Bulge in late 1944 and early 1945.