150 years ago, there were perhaps one tenth as many English speakers as there are today. If you write a book that appeals to one percent of the English speaking world, that will give you ten times as many potential readers today as you would have had in the mid-nineteenth century. Which means that even if you only charged half as much (in relative money), you'd still earn five times as much for the same amount of cleverness and creative effort. (And that doesn't even get into modern publishing technology, international markets, or the increase in literacy.)
This seems very weird to me. It's not like an apple, where you can make more money selling apples to a thousand people than by selling them to a hundred people, since in that case you would still need a thousand apples. But you only need to write one novel. The exact same thing is worth more, not because of scarcity (there is no limit to the number of copies that can be made, especially if it can be digitized) or because the product has become any more useful to anyone. But just because there are more people around.