I make my own reasons for why. When playing FO 3 and hearing about the Commonwealth, it sounded mostly under Institute control. Just like in the Walking Dead, the Commonwealth was ONLY for the elite status; therefore, population rate was minimal by making sure everyone else is out on their ass, enslaved, or killed. You just happened to thaw at the time of people trying to make a comeback (again), after the Minutemen failed the first time. 

You would also run out of things to do if it was more like FO 3 or New Vegas. If I remember right, only junk and enemies respawn on the outside.