(make sure to thank me)
The main reason of the senate's rejection of the treaty was to avoid the "foreign entanglements" of the League. However, a more sinister reason might have been that the Republican dominated Senate disliked President Woodrow Wilson (who refuse to have any Republican leaders with him on the tripe to Versailles) and wanted his plans for the treaty and the League to fail.
In other words, the main reason why the US Senate did not ratify the Treaty of Versailles was the League of Nations. There were serious concerns that the League would erode US sovereignty and pull the US into further wars that were not in its interest. They felt that US might be forced to go to war in order to protect other nations that were invaded.