2 years can be a long time in the entertainment industry so there are always concerns that fans will move on without regular content from their faves. Then when they come back they might not be able to attract those fans back. From what I recall it's only recently that members have started releasing content while they're enlisted to keep fans entertained.
Plus during the enlistment period members have more time to focus on solo activities that can often be hindered by group activities.
So enlisting together is always a discussion, but enlisting separately has more benefits.