I mean sure, Scala is a great language and has a lot more in common with Rust than most languages, but it's fundamentally designed for a different role than Rust. Rust is designed as a "systems language", whereas Scala's dependence on the JVM is a non-starter for such a role.

You could say instead that Rust is a Scala-inspired systems language. Not just Scala, obviously, but by functional programming concepts like algebraic data types and type inference. Rust isn't even really a functional language itself; it's still an imperative language.

Because performance is a P0 concern to a lot of people. Rust (and C++) has performance as a primary goal, Scala does not.