Swift is 6 years old, Rust might have been popular among programming language connoisseurs at the time, but it wouldn't have been that silly to expect it to be a fad.
I see more people and companies adopting Rust since last year after Rust 2018 version with async support.
Rust hadn't hit 1.0 6 years ago and it changed a lot leading up to 1.0
Another factor here is that Chris Lattner, creator of Swift, used to work for Apple, with a high-ranking job, he was directing a whole department. He's since left Apple a few years ago. No longer having the creator of Swift championing its use at Apple opens the door to alternatives. I haven't used Rust myself, but it looks like a solid language for systems development.