WWI was won by the Allies mainly by the fact that the US, with its huge population and industrial capacity, entered the war.
Up until the US entered the war, the war had been a real stalemate. The Western Front, where most of the important fighting was going on, had not moved any significant distance in years. This showed that the Allies and the Germans were essentially deadlocked.
When the US entered the war, this all changed. The US had the potential to send millions of men and huge amounts of materiel into the war. This convinced Germany that it could not win.
The US entered the war due to its economic and political/social ties to England. The US conducted huge amounts of trade with the British and had deep historical ties to Britain. This, combined with the German submarine warfare, brought the US into the war.
So, the Allies won because they were able to get the US on their side. The economic and political/social ties between the US and Britain (and to some degree France) brought the US into the war, changing the strategic balance.