Many Americans felt that the treaty was unfair to Germans. They also thought France and Britain were financially benefiting themselves at Germany's expenses and that America was helping them (which was not the case, but many Americans believed that).
Also, since Wilson (the president at the time,) was the head of the Democratic Party and he didn't consult the Republican Party while approving the treaty, the Republicans became upset. Since they controlled Congress at the time, they criticized Wilson and ultimately, Congress rejected the treaty.
