Why did Cosimo I receive the title grand duke of Tuscany in 1569?

On August 27, 1569, Pope Pius V conferred the title of grand duke (granduca) of Tuscany upon Cosimo I as a gesture of favor. By this time, Cosimo had successfully brought nearly all of Tuscany under his control, including the republic of Siena, which he had conquered in 1555.1

Although the title was granted by the Pope, it was not recognized by the Habsburg powers or other Italian duchies at the time. To further gratify the Pope, Cosimo married his long-time mistress, Camilla Martelli, in 1570.1