Because books were very expensive in the 18th and 19th centuries and the price of a book was out of the budget for the vast majority of people. Lending libraries existed, but you had to pay for them, too, and many could not afford it. Newspapers were inexpensive enough, and the print small enough, that more people could afford it, but only if they paid a little each week or month. For people who were a little better off, they sold novels in parts--one part every few months. The paper quality was higher and the paper size was that of a book. After all of the parts were published (typically for Dickens, in 20 parts), you could collect them together and bind them into a book, if you had enough money, since book binding was expensive, too. Until the early 19th century, books came without a binding. Since only the wealthy could afford books, they would buy the book and then pay to have it bound in a style to match their home's decor. The lending library would leave them unbound in boards.