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...

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.