There is an old cliché that says that the winners get to write the history. That is the major reason why the Treaty of Versailles mainly blamed Germany for World War I. Since Germany lost, its version of history did not get written into the treaty.
It must be said that there is at least some justification for saying that German actions led to the war. Germany was the main country that was unhappy with the status quo before the war. It was pushing for its “place in the sun.” Its aggressive attempts to become more powerful scared other countries. To this degree, it is fair to blame Germany for the war.
But it is not fair to say that the war was all Germany’s fault. The problem is that fairness does not always enter in to things like treaties. Treaties are dictated by those who have the power to do so. The Allied Powers had the power after the war. Of course, the Allied Powers felt that the war was the fault of their opponents and not their own fault. Therefore, they wrote the treaty to reflect this idea.
So, the main reason why the treaty was written in this way is that the Allied Powers had the power to write it to reflect their vision of how history had unfolded.