I love Swift and it's potential for use cases beyond just iOS ecosystem, but I think Apple never fostered or encouraged the open source community to involve in its development.
It's another classic example where it's simply not enough to open source something, you have to really allow and encourage the developers to contribute to increase the adoption.
I'm excited about a few projects bringing Swift to other platforms. SwiftWasm is surprisingly painless to get working [1]. Tokamak is an early stage SwiftUI-compatible framework for building web apps [2]. It also has the goal of supporting other native platforms and to that end there is early work on a GTK backend. I have no affiliation with these projects, though I would like to contribute to them at some point.