Because in the past, after the opium war, China had to let the western countries set up embassies in Gulangyu. There are many old consulate building, including German, American, Japanese to name a few. There are numerous western buildings in the islet and. The old Japanese embassy is now an Organ Museum, collecting old organ from different parts of the world.
I have been myself twice and stayed on the island for a few nights. Highly recommended. I will post my photos once have enough time to sort it out.