Wilson's efforts to keep the United States neutral failed, simply because the United States could not afford to remain neutral.

American businesses had benefitted from the stimulation brought about by trade with Europe since 1914, c.2-2.5 Billion dollars of which had been with the Entente (primarily Britain and France), and c. 300 million with the Central Powers (primarily Germany). Considering that America had been experiencing an economic 'slump' prior to the war, it was only natural that such dealings would take place. Financially however, the United States was quite heavily invested in Britain and France, and by 1917 it had reached the point that without further aid, the Western Allies would be in dire straits, especially as it became more apparent that Russia (the Entente's largest member) was descending into disorder.

That said, don't mistake this for the spiel about 'Merchants of Death', 'betting on war' because 'War is a Racket' (it really isn't). Such concepts are far too simplistic, and ignore the valid reasons for American intervention.

By February 1917, when the United States officially declared war, the war had changed. Britain, France and even Italy were becoming more democratized, and Russia had become nominally democratic as a republic. On the opposite end was Germany, a military junta, Turkey, also a military junta (a genocidal one to boot), Austria-Hungary (the 'Prison of Nations'), descending towards anarchy, and Bulgaria, an autocracy. Germany had already been carrying out acts of espionage on US soil, and this included acts of terror like the 1915 Vanceboro Bridge Bombing. German U-Boats had attacked neutral shipping, killing civilians, and were poised to do so again (and did). The clincher was the Zimmermann Telegram; Germany reaching out to Mexico (it already had spies there, and had provided some weapons) and Japan to attack America, distracting it from Europe. This was the final straw, as the past years had more or less resigned many including Wilson that in order to ensure a peaceful order in which the United States would be secure, they could not risk a German victory, allowing them to establish hegemony on the continent. It would also need to be a peaceful order that America had a role in creating, and that's more or less what they got in 1918-19.

Now that they could say that the Germans had provoked them (they had), and that their Allies were at least vaguely democratic (they were more than just vaguely by this point), the United States could enter the war with a clear conscience, although not without great reluctance.

TL;DR: The challenges and threats that the War brought about could no longer be ignored. The United States had to acknowledge that neutrality wasn't working.