The background for this is that most existing UI toolkits are a poor fit for Rust.

Rust doesn't like having shared mutable state, but event-based UIs have a global event loop and can mutate anything at any time.

Rust works best with strictly tree-shaped data structures, but event handlers turn trees of widgets into arbitrary webs with possibility of circular references.

Rust prefers everything thread-safe, but many UI toolkits can't even be touched from a "non-main" thread.

Rust's object-oriented programming features are pretty shallow, and Rust doesn't have inheritance. That makes it awkward to model most toolkits that have deep hierarchies with a base View type and a dozen of Button subclasses.

So instead of retrofitting mutable single-threaded OOP to a functional multi-threaded language, there's a quest to find another approach for UIs. This change of approach has worked for games. Rust wasn't nice for "class Player extends Entity" design, but turned out to be a great fit the ECS pattern.