Because it wanted to protect it's economic interest and reserve it's military resources and superiority.
In the months leading up to World War One, the US maintained neutrality as President Wilson declared as it did not want to entangle itself with the other countries or allies, especially when given how chaotic the situation are. It did not wanted to be invovled in the war, given how it would inevitably weaken the state. The reserved resources would give it superiority when the others were drained out from war.
Also, US used to trade with both the Central Powers and Allied Powers, therefore it wanted to keep trading with both thus not wanting to choose side in the war.
Hope it helps!