What would three different schools of International Relations (realist, liberal and identity) say as to why WWI started in 1914 and not before that?
Realists would say that the war did not start before that because none of the countries felt that its security was being threatened enough to make war worth it before 1914. Realists believe that countries always act to preserve their security. If they don't go to war, it's because their security doesn't demand it.
Liberals would say that misunderstandings had not yet gone far enough to cause wars. Liberals say, among other things, that war is caused when the various sides don't communicate and therefore don't really understand each others' needs and concerns. When this misunderstanding gets bad enough, war can result.
People who focus on identity would say that, before 1914, no country had done something bad enough to make another truly hate them. Before 1914, the countries might have disliked one another, but they did not see the other countries as "bad enough" to deserve war.