I think it's partly architecture compatibility. The SIMD functions exposed by Rust at the moment are all CPU-architecture-specific, but Rust code is generally expected to work seamlessly cross-platform.

I believe there are plans to expose higher-level functionality as a safe interface at a later date (the API design work just hasn't been done yet). For now you can get this functionality as a 3rd-party crate https://github.com/AdamNiederer/faster