I'm not sure this is why they did it (I doubt it is), but reading through the documentation I saw that it's playable on Linux, Mac and Windows. I don't know when I've seen an MMO that's not browser based that was playable on all three platforms. C# engines usually ship to Windows and Mac - though, in saying that, I'm not sure if that's engine enforced or developer choice.

The most likely reason they chose to do it in Rust, though, is because they wanted to and no one else had.