Mark C's question rang a bell about something I'd read here on L&T awhile back regarding the practice of creating multi-volume works for circulating libraries. I dug around in the archive and link Henry's post below.

It seems one of the factors in publishing these multi-part volumes was to make it more cost-effective for the circulating libraries, by having subscribers "simultaneously" reading a novel, albeit in different parts.

I suspect that this, in addition to the reasons Mark guessed at and Dee also laid out, made it a financially logical thing to do in those times.

(And Henry's archived post indicates that once a popular book has run it's course, cheapers single-volume editions would come out later... perhaps like the modern day equivalent of a hardcover followed by paperback?)