Question: how and why were serial novels a phenomenon in the nineteenth century?
Answer: improved economics of distribution, the rise of literacy, technological advances in printing

Serialized fiction surged in popularity during Britain's Victorian era, due to a combination of the rise of literacy, technological advances in printing, and improved economics of distribution.  Most Victorian novels first appeared as installments in monthly or weekly periodicals. The wild success of Charles Dickens's The Pickwick Papers, first published in 1836, is widely considered to have established the viability and appeal of the serialized format within periodical literature. During that era, the line between "quality" and "commercial" literature was not distinct. Other famous writers who wrote serial literature for popular magazines were Wilkie Collins, inventor of the detective novel