Why was Italian used as the universal language of music?

Initially, the innovators in music (beginning in the 16th century) were Italian. When a northern prince, such as the Archduke of Austria (who was usually the Holy Roman Emperor into the bargain) wanted muscisians who were conversant with the newest trends in music, he would import Italians. The invention and development of the violin in Italy greatly increased the ifluence of Italian composers. At the end of the 16th century, Montiverdi wrote the first operas, which were, understandably, in Italian.

So, even when a German or a Frenchman learned formal musical rules in their home country, the tempo notations and descriptions of established forms were in italian. Later, composers began to use their own language for tempo changes and descriptions of passages, such a Beethoven in his descriptions of the movements of his sixth, "Pastoral" symphony.

Italian dominated music in the realm of notation because Italians dominated the world of music.