You also have people who do not love manga that much, they like some of them, and online reading is enough for them.
Also, no download to manage, less exposure to legal trouble etc...
It also is provides a good way to discover some manga.

For example, I try to follow Initial D. The manga isn't stellar in anything from scenario to art quality and is mainly composed of two cars chasing one another, most pages are filled with that. So there isn't much interrest in downloading in that case.