Why did attempts to break the stalemate on the western front generally fail?

During World War I, the war's Western Front turned into a stalemate after initial advances by the German forces for primarily one reason. That reason is this: defensive technologies and tactics in the early 20th century were more advanced than offensive technologies; thus, offensive breakthroughs were either not possible or, where achieved, could not be sustained at length. Significant victories could only be achieved where some external element (e.g., egregious leadership mistake, troop demoralization, lack of supply) occurred, but these did not occur on the Western Front until 1918.