Why did the War not end before Christmas 1914?

The first World War started in July-August 1914. There were two major groups with many powerful countries involved. The countries were Germany, Austria-Hungary and Italy on one side and Great Britain, France and Russia on the other side.
Much before the war started, Germany had formulated Schlieffen Plan first in 1905 and modified it in 1911. The plan had assumed that if there was any outbreak of war in Europe, France would automatically join Russia and Germany would have to fight both of them in Western and Eastern Front. The plan had therefore proposed that the bulk of German forces we…