I think they wanted more control over the GUI, so the best answer to that is build something that you have 100% control over. In one of their first blog posts they explain more in deep the reasons.
> Inspired by the gaming world, we realized that the only way to achieve the performance we needed was to build our own UI framework: GPUI. [0]
GPUI is not widely used because they only recently started publishing in crates.io [1] and still AFAIK their documentation is not that great yet, I suppose people will start using soon enough it, I was thinking on using it over Iced that was the other GUI that I feel that have future (because it's used by Cosmic Desktop).
> Inspired by the gaming world, we realized that the only way to achieve the performance we needed was to build our own UI framework: GPUI. [0]
GPUI is not widely used because they only recently started publishing in crates.io [1] and still AFAIK their documentation is not that great yet, I suppose people will start using soon enough it, I was thinking on using it over Iced that was the other GUI that I feel that have future (because it's used by Cosmic Desktop).
[0] https://zed.dev/blog/videogame [1] https://crates.io/crates/gpui/versions