Emil had already started egui long before we started Rerun. The immediate mode paradigm of egui also fits very well with how we wanted to archetype Rerun. In addition to that the GUI framework story in Rust is still quite immature which means you often need to be able to make changes to the actual GUI framework if you want to create a "cutting edge" UI heavy product in Rust. Not surprisingly you therefore see other Rust based apps build their own GUI frameworks for that same reason. There are some promising initiatives in the Rust world but we'd still make the same decision to build on egui today if we were to start from scratch.