Carolyn Abbate, co-author of A History of Opera, explains how the genre has evolved:
I have teenaged sons, and I ask, "What’s the difference between a [popular music] concert and the opera?" They say the difference is at the opera you have to be quiet and you can’t move. But that wasn’t always true. Two hundred years ago no one was required to be attentive and focused. It was routine for people to talk amongst themselves. They could go in and out whenever they wanted. And eating was allowed in boxes, as was gambling and chess playing. It was a social occasion that happened to have something going on at one end of the room that you could pay attention to if you wanted to.
She blames Wagner for the current state of affairs:
He was the first to declare that the auditorium had to be pitch dark. At Bayreuth [which opened in 1876], he imposed all kinds of other religiosity on the experience — like having to be absolutely quiet. He talked about how people had to be utterly attentive to the stage world and not each other. He erased the social function of opera.
More on Abbate's book here.