Why did President Wilson find it so difficult to keep the U. S. out of world war 1

The two biggest reasons was the Lusitania, also the Zimmerman telegraph.

The Lusitania showed that the unrestricted submarine warfare would go after any ship passenger or military.

Then the Zimmerman telegraph from Germany basically asked Mexico to attack the U. S. and Mexico would receive all the land they lost in the Spanish-American war (Texas, Arizona, New Mexico).