I've constantly seen UI questions rejected on the User Experience Stack Exchange and told to go to the Graphic Design Stack, which I'd be inclined to argue is different and you rarely see UI questions on there.
UI is interlinked with UX so I dont know why the questions get rejected so much, but it is also its own profession too.
So with this frustration, I tried to set one up on here but it got shut down due to it being a 'duplicate of the User Experience Stack Exchange'. So you can see my conundrum and frustration. My example questions probably weren't the best but they where up for discussion and improvements.
This sounds like im a bit bitter! I'm honestly not, im just trying to solve a problem I have come across as a UI Designer wanting to put my questions out there.
You can see the original proposal here: https://area51.stackexchange.com/proposals/118967/user-interface