First, we must acknowledge that it is not always possible to know what causes changes in society and governmental sytems to occur.  We can only speculate using as much information as we have.  The general consensus is that dictatorships tend to arise in times of stress in a country.  When a country is in crisis, it is more likely to be willing to accept a dictatorship.  World War I and the events of the ensuing years helped to bring about crises in a variety of countries, leading to the rise of dictatorships.

The two most brutal and egregious dictatorships in Europe after World War I (or perhaps in any place and time) were Nazi Germany and Soviet Russia.  Both arguably came about because of stresses related to WWI.  The Soviet Union’s dictatorship was formed directly by WWI.  The war went so badly for Russia that Russians overthrew their ruler and eventually installed a communist system.  They were willing to accept a dictator because their country was in chaos due to the failure of the previous government in WWI.  Germany accepted Nazi dictatorship because of the aftermath of WWI.  Germans were very unhappy with the outcome of WWI.  They felt unjustly punished by the Treaty of Versailles.  Their economy, by 1933, was devastated by reparations that they were forced to pay and by the Great Depression.  This crisis led them to accept dictatorship.

Thus, it was crises arising from WWI that are generally blamed for the rise of dictatorship in the years following the war.