Presumably because there aren't many mature Rust GUI options at the moment. Egui is pretty good for what it is, but it's "immediate mode" which comes with some restrictions. Slint seems promising but the developer experience was a little rough when I tried it a couple months ago. Not sure the licensing is compatible with this project either.

You can have much better OS level integrations with Electron (e.g. filesystem), also assuming that you want to reuse as much code as possible.