Because the alliance said they stood by each other only in attack.
Italy had been allied with the German and Austro-Hungarian Empires since 1882. Italy had its own designs against Austrian territory in the Trentino, Istria, and Dalmatia, and maintained a secret 1902 understanding with France, which effectively nullified its previous alliance commitments.
Italy refused to join Germany and Austria-Hungary at the beginning of the war because their alliance was defensive.
The Austrian government started negotiations to obtain Italian neutrality in exchange for French territories namely Tunisia, but Italy joined the Triple Entente by signing the London Pact in April and declaring war on Austria-Hungary in May 1915; it declared war against Germany 15 months later.
To learn more, see www.newworldencyclopedia.org - World War I and mrmasonworldhistoryb.weebly.com - The Great War Begins.
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