There are only two permanent locations and a couple of temporary islands available in the game simultaneously. If you want to open new island, you have to "close" the one you have already completed. This procedure is designed to keep your game clean and stable. Every new item, decoration, and completed quest makes your game state bigger.

The bigger your game state is, the more virtual memory (RAM) it requires, which ultimately means that if you want to keep all the islands, you’ll need a spaceship worth of RAM. We want to make the gaming experience pleasant to everyone, including the players with older devices. That’s why we need to "close" the island to get to the next.