I love this style. Is it just me? Dickens, Stevenson, ...
I would rather ask the opposite : why the need to adopt a "modern" style (well except in fantasy books, I guess).
Most authors were paid by the word at the time.
One classic example would be Charles Dickens, whose novels started out as serialized chapters in newspapers. While it doesn't take away from him being an excellent writer, if you ever wonder why some of his stuff seems to have filler chapters... well, now you know.
Likewise why were German philosophers and thinkers so apt to write dense and convoluted sentences? I'd like to know.