People actually can't get into their homes today because too many people are on islands. Islands are affecting houses. This strongly implies that the islands share the same resources as the homes/neighborhoods.
Before today, the prevailing argument for "why can everyone get an island, but not a house?" was that islands were wholly divorced from the housing setup. "It's a new system!" they said. "It's not anchored to the housing system!" they said. "It utilizes different resources, less resources!" they said. Yet here it is, clearly sharing the same resources as the homes and neighborhoods, since too many people in islands = people can't get into their houses.

Days like today are why I want Square to actually say why we can't have instanced housing. Players can guess at the reason all they like, but it's obvious that none of us know a thing about the inner workings of the game, and square's limitations seem to change with the wind. "Not enough resources!" say the people who don't even represent the company or know what's going on behind closed doors. You say "stop asking for instanced housing." I'd say "stop pretending to know why Square can or can't implement content." Let the 2 billion dollar company speak for itself.