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The borrow checker.

UIs tend to have designs that require graph based structures and self referencial references, so in the end you get either lots of Rc<RefCell<>> and clone() calls, or have to bind the whole stack to fit into Rust's memory model.

Have a look at,

https://raphlinus.github.io/rust/druid/2019/10/31/rust-2020....

https://raphlinus.github.io/rust/druid/2020/09/28/rust-2021....

Many UI toolkits use shared ownership and callbacks, and that's annoying in Rust. Some bindings make it work. There's a bunch of folks working on interesting architectures. But the classic styles just clash with Rust's default way of doing things.

Multi threading sanely requires Arc, and reference counting is a crappy form of GC.

If you have enough threads passing stuff around and can deal with the ~50% performance loss between Java and Rust, it's much easier to use a language with GC.

There's talk of adding a quality GC to rust and dedicated GC types. If/when that happens, Rust will be a contender for a bunch of higher level systems