A significant property of Isalnd Sanctuaries is that it's not just instanced: it's isolated.
You can't use or make gil there, because it has its own currency.
You can't use or make items (e.g. Thav onions) useable in the "out-world" there.
You can't trade the items you make or use there with other players, who can only interact with your Island Sanctuary as look-don't-touch tourists, so no gil-for-IsleBucks trading.
And odds are, your out-world crafter and gatherer progression will be useless on the island.
So any features (or bugs, which is often the same thing) in Island Sanctuaries won't have any effect on the non-Island game economy. It won't affect supply or demand for anything.
The only link between the out-world and the Islands is, apparently, your minions, who can roam the Island, and your FC and friends list, which determines who can visit it. This means that it is very unlikely that Islands will screw up progression or screw up the game economy. And don't underestimate how important not screwing up the game economy is.
So why does this high degree of isolation make the Islands easier to implement than instanced housing?
Because in complex systems, a big source of gotchas, and often the biggest source of schedule slips in development, is unexpected interactions between components. The more isolated the components are, the less you have to worry about it.
Furthermore, in software development, the bigger the team that has to work together is, the more problems you're going to have because of miscommunication and interpersonal conflict. But because Islands are so isolated, the team that is coding (and doing the art) for it is probably working in isolation itself.
So, you might ask, why not do a similar thing with housing? But would most people want that: housing with only furnishings and goods you could not buy for gil, could not craft and sell for gil, and could not trade with other players? I wouldn't.