There are a couple reasons why people stopped. For example, there are a few series that have spanned years and it looks like they are nowhere near completion. The characters are essentially just one or two age older than when they started but the readers have aged a bit. People eventually find the story boring or lost track of the development.
I definitely agree that this is a phenomenon happening currently... it could also be interesting to include an analysis of manga that people have stuck with, and reasons why readers made the decision to stick with those manga, even though many of these also took years to complete (Naruto or One Piece, for example). – ees6 years ago