Andrea Broomfield argues in Food and Cooking in Victorian England that toast really only became a common thing in the nineteenth century. This is not to say that no one ever toasted bread before that--obviously, people have been eating bread for at least 10,000 years and much of it got toasted at one point or another. She argues that it was not until about 1850, however, that toast became a form of eating bread both common and meaningful.

Now, it's impossible to prove a negative, so I cannot say with certainty why people did not start toasting bread earlier. I can, however, point out that toasting bread has a number of limitations. One is it's time-consuming. It must be served hot to be good, and if you're doing it by hand--with a toasting fork, as it was first done in Victorian England--that means that it would be very last thing the cook would do before serving a meal. Moreover, if you're doing individual slices on toasting forks, you can't do very many at once. For people used to untoasted bread, this might seem like an unnecessary food preparation step.

For the Victorian middle class, however, these limitations operated as markers of status. For them, Broomfield argues, toast at breakfast in particular came to signify a well-run household. It required a competent cook and a tight schedule to get the large, hot breakfasts that Victorians like onto the table with hot toast. A family that could manage that, then, was clearly good at running its household--at least in so far as that family hired a good cook.