Never let it be said that Turbine isn't mindful of their fans. Forum moderator Scenario was there with a quick reply. To wit, "As housing receives continued exposure, we will be looking over our metrics and seeing how much more we can add while still providing an environment that performs adequately (so far we don't have anything to worry about but we've never instanced landscape in this fashion before, so we wanted to be cautious at first). At the soonest, you may see some new NPCs added to neighborhoods in Book 12 - but I make no guarantees!"
We can't fault them for wanting to be cautious, but we want more surly dwarf milkmen! But anyway, there's your answer.