We have to design our games to make players want to keep paying, and keep coming back, at minimum cost to us the service provider. This is one of those things so obvious on the face of it that people tend to miss the point. In a subscription-based model, the ideal online game is one where the player keeps paying to never log on. A session-based model means you need them to log in to play. You have to count on players to take initiative.
Most players have trouble remembering to watch their favorite TV show.
If you want to trust players to reliably do something that is not woven into their daily lives and that they have to pay for on top of that, well, they probably won't. I don't know very much about session-based models except that they aren't something that interests me very much as a designer, so I won't dwell further on that. I am sure that there are some of you out there who are shaking your heads in dismay at how naive ve I am about it anyway. So retention is a key factor. But it's not the only factor.