America entered WWI for political reasons.
Political:
Two important political factors contributed to the entrance of America into WWI: the sinking of the Lusitania and the Zimmerman Telegram.
- The Lusitania, claimed to have been carrying arms, was sunk by a German U-boat, killing many, including 123 Americans.
- The Zimmerman Telegram was a telegram from Germany to Mexico intercepted by the US asking for Mexico to join on Germany's side and promising they would let Mexico keep the territory they won back from America.
Economic:
America was already making a huge profit over selling arms to both sides, and entering the war would change that. If America entered the war, they would not be able to sell to both sides and would make less money. In addition, then they would have to spend money on manufacturing their own arms.
TL;DR political reasons.