First question: How are you treating different groups of humans?
If you're looking for a reason to push different races into isolation.... well, there's always the zombie apocalypse. The demon invasion. The great insect revolution. That is, fill the space between them with monsters, and give the elves a touch of magic bugspray that repels them from their trade caravans.
But also, you could think about toning it down a little. Even if elves are the only traders plenty of people would still want to travel with them.
Finally, forget about the backdrop for a second. The real messages in your book are focused on you main characters and your antagonist. What is your story here? And how does all the isolationism play into it?
If you're looking for a reason to push different races into isolation.... well, there's always the zombie apocalypse. The demon invasion. The great insect revolution. That is, fill the space between them with monsters, and give the elves a touch of magic bugspray that repels them from their trade caravans.
But also, you could think about toning it down a little. Even if elves are the only traders plenty of people would still want to travel with them.
Finally, forget about the backdrop for a second. The real messages in your book are focused on you main characters and your antagonist. What is your story here? And how does all the isolationism play into it?