Why it had to be Swift

Hardly. With full optimization Rust doesn't lose type safety while Swift does. There are completely different tool-chains and philosophies at play here. Swift does barrow some philosophical ideas from Rust (which Rust took I guess form Haskell), but the execution of those ideas is botched.

Likely what happens is Swift is just transformed into identical Obj-C byte-code and ran though the same toolchain. Instead of developing its own tool chain so it could live up to this lofty goal.