The tl;dr is: because you want native performance and don't want to focus on blitting pixels or terminal internals.

Selkie has an event/effect model similar to Elm & re-frame, you just declare your widgets and renders/updates are handled by the lib.

As for why Raku:

- grammars allow for easy parsing of complex DSLs

- supply/tap are a natural fit for the event/effect model, asynchronous thread-safe programming

- roles make composing widgets simple

- it's fun to write :D