Bungie designed the game to be played continuously, like an MMO. After a month or two (if that) of burning through all the PvE content you'll be done with it and move on to another game. The people who stick around are there for the PvP. They probably still run their Nightfall each week and maybe even a raid—but just to stock up on infusion fodder for their PvP gear. PvP is what has kept the same people playing during the long stretches between the major expansions.
Bungie's attitude in how they approach balance reflects this. PvP is clearly the priority, and PvE often suffers as a result.
I think the reason PVE players don't stick around is because there isn't much content to make them want to stick around. Rise of Iron, besides the raids, was pretty barebones: 5 missions, most of which were short and lackluster, and a couple new strikes.
Part of the problem might also be that the play areas don't have much to explore. A more open world format might help with that.
Focusing more on PVE will make people want to stick around for PVE
That said, there will still be content draughts no matter what, but they could minimize this. I actually think it would be interesting if they did something where they released X amount of story missions all at once, but then released one mission per week after that, similar to how a TV show releases episodes. Might help generate hype and get people excited about the story, etc.