Why was world war one not over by Christmas?

It is thought that the reason the first world war was not over by Christmas, was because Germany had underestimated everything, everyone's capabilities, and to a degree even themselves. My thoughts are that this was the reason, although people argue that there were other causes. I think that this one reason links into every area of why the war took so long to finish. So you could say that not only is Germany underestimating everything is a reason, or even the main reason but it is also the only reason for the war as well.

The Schlieffen planWas Germany's plan to win a quick war. This plan was not looked into enough, and yet the Germans were so sure that it would work. The Schlieffen plan was Field Marshal Alfred, Graf von Schlieffen's (February 28, 1833 - January 4, 1913) plan to prevent Germany from fighting on two fronts.

British autos with cannon in France (LOC)

No Mans Land, Flanders Field, France, 1919 (LOC)

[Opera House staircase, Paris, France] (LOC)

Germany would do a lightning attack on their western neighbour France, through Belgium because they thought that Belgium was so small and weak that they would lose hardly any troops. They would then go round the front line of France and head for the capital Paris. Alfred Schlieffen was under the impression that France would fall aprat and surrender if the symbol of their country was destroyed; Paris. Alfreds idea was to defeat France in six weeks, this was thought to be the the time it would take for Russia to mobilize their army. After the speedy defeat of France, the German troops would turn to concentrate all actions on their Eastern neighbour; Russia, Russian mobilization was thought to be extremely slow, because of their bad railways. The Plan was to send send 90% of the German troops to France and 10% to Russia.

ChangeThe plan was so near...