Question

Why do you think the stalemate continued for so long

Answer 1

 World War I changed the face of warfare. It was caught between the old tactics of set-piece battles and gentlemanly tactics and the idea of Total War that was beginning to be used.
Neither side really knew how the new technologies would change the way that war worked, or how high the carnage would be. Defensive weaponry had advanced far beyond that of offensive, so both sides could dig in and hold off large attacks, but neither could really advance very much.
Finally, the mindset of the people had to change. Up until that point, war had been thought by many to be nothing more than a grand game (British troops in the first months marched across the battlefield kicking soccer balls, thinking that it would be fun to win the war, they were slaughtered en masse). That idea was detrimental to running a real war.