We know Utopia is an island. In fact, the first king, Utopus, physically turned it into an island, so if you're thinking that's got to be important, you're on the right track. Check it out:
"They say (and the appearance of the island confirms this) that their land was not always an island. But Utopus, who conquered the country and gave it his name […] also changed its geography" (2.43)
Islands have a long tradition of being places where unusual, magical, even scary stuff happens. Think Homer's Odyssey, where Odysseus encounters all kinds of fantastical stuff on different islands he accidentally visits: witches, giants, dead people, monsters. So, if you're going to invent a undiscovered, unconventional society somewhere on the planet, it's going to be an island.
Islands are also naturally isolated, so they can exist just outside the bounds of authority and away from the influence of the rest of the world. Think about how important that fact is for the whole social organization of Utopia, which has developed entirely different customs and attitudes to living than anywhere else.
That's it. We're moving to Hawaii.