The Treaty of Versailles contained what it did because those provisions were what the victorious powers wanted. The Americans, the British, and the French in particular put in provisions that they wanted.
The provisions of the treaty were the result of negotiations between the victors. The US, for example, wanted a very lenient peace. They wanted a new world order that would be more conducive to peace. This resulted, for example, in the inclusion of the League of Nations in the treaty.
However, the French and the British were not willing to be so lenient. They wanted to punish the Germans and to ensure that Germany would not be able to threaten them again. Therefore, the treaty included such things as the large reparations that Germany had to pay. It included things like the demilitarization of the Rhineland and the tough restrictions on Germany's military.
In these ways, the Treaty of Versailles contained what it did because that is what was agreed upon by the victors in consultation with one another.