Ok, here's my theory. The explosive success of Italian opera in the Baroque occurred because of the serendipitous confluence of two factors.
1. Throughout its history until near the time of Rossini's retirement (coincidence?) the church forbade women to sing in the church. In Rome this prohibition extended to all other public venues as well. From the mid 16th century this gap was filled by castrati who were limited to singing the multi-voiced counterpoint of Lasso and Palestrina. It is my understanding that they ornamented in this style, too, though it is a bit hard to imagine. Despite that, the style didn't allow sufficient scope for their virtuosity. For a castrato singing is his life.
2. The Florentine camerata invented a medium of theatrical solo singing based on the classics.
The two streams met in Rome in the 1620's, and a colossal explosion occurred. Castrato virtuosity met the perfect context for its display, and the rest is history. For the first 200 years that was the fundamental purpose of opera everywhere except France where castrati were never fully accepted. Did I say that strongly enough? I am proposing that castrati are the reason for opera, and not opera the reason for them, as I had always thought.
That's my theory.