I think the premise of your question is wrong. From the mid-1800s there was an ever-developing Armenian national liberation movement that (similar to the successful Greek national movement) challenged Ottoman rule. As Armenian nationalists grew in number and strength the Ottoman state frequently clashed with them.

In 1896 the Ottomans massacred approximately 200,000 Armenians during the Hamidian Massacres. In 1905 the Armenian Revolutionary Movement tried to assassinate the Sultan. It was far from a peaceful coexistence, and WW1 just intensified the pre-existing issues.

As this map from 1899 shows, the Ottoman treatment of Armenians was seen as a stain on the Empire's reputation well before the 1915 genocide.