Why was the great war so much more protracted than the wars of the 19th century?

I'm not sure I'd say that the Great War was more "protracted" than 19th century wars.  The Great War lasted four years and some months.  In the 19th century, the Civil War in the United States also lasted over four years.

The biggest difference in the Great War, early in the 20th century, was how massively destructive it was.  Heavy industrialization and militarization by European nations made for a conflict that involved many nations with enormously devastating ways of inflicting damage on one another.