It doesn't have to be Rust. But for all the people who insisted for more than a decade that GCs and VMs don't necessarily compromise performance of practical applications, there has never been an acceptable browser engine in C#, Java, D, Common Lisp, Go, OCaml, Haskell or any of the others. Meanwhile Apple uses Objective-C with its ARC (and later Swift, which is even more like Rust) for everything and it was great.
So the community tried to make OCaml with the memory model of ObjC and as much backwards compatibility with C as they could muster. In context, this doesn't seem like a weird strategy.