The statement that best explains why Russia left World War I before it ended is: The damage of the war caused an internal revolution.
The tremendous losses and hardships caused by the war, combined with economic struggles and social unrest, ultimately led to the Russian Revolution in 1917. This revolution resulted in the overthrow of the Tsar and the Bolsheviks seizing power, which led to Russia signing the Treaty of Brest-Litovsk with Germany in early 1918, officially withdrawing from the war.