As I said, it's a massive undertaking. The minimal acceptable toolkit (to say nothing of competing with GTK or QT, just getting the minimal toolkit that users will tolerate) is an immense amount of work. Quite a lot of it is essential complexity, the things that textboxes do that are simply expected and are true representations of complexity, not accidental artifacts of the paradigm used. Functional programming may or may not help with accidental complexity but nothing can duck the essential complexity. I'd estimate that even a minimal toolkit would be on par with the time invested into GHC itself, and to seriously compete, tack a couple of orders of magnitude on there.

In a world where everybody implements MVC web framework in their favorite language, reimplements window managers in their favorite language, and reimplements all manner of other software simply because it wasn't in their favorite language... there's a reason all the communities just bind to GTK, QT, and wxWidgets and don't reimplement widget frameworks from scratch in their favorite language, even for languages much better funded and bigger than all the functional programming communities put together. Just the bindings can be major and in some cases unsustainable efforts for smaller communities.