During WWI, what contributed to neither side advancing in battle on the Western Front?

As the Germans and Allies built trenches Neither side was able to defeat the other.
A stalemate developed on the western front for four main reasons, one being that the schlieffen plan failed, another reason was that the French were unable to defeat the Germans completely at the Battle of the Marne, another reason was the “race to the channel” and the last reason was the defending positions far easier than attacking.

-In their search for a weapon that could break the stalemate on the western front, generals turned to a frightening new weapon-poisonous gas. On 22 April 1915 near Ypres, the germans released chlorine gas from cylinders and allowed the wind to blow the thick, green vapor across to the Allied trenches.