In addition to what others have said, they are different types of GUI systems. As an immediate mode UI, the logic behind what gets drawn to the screen in Nuklea

In addition to what others have said, they are different types of GUI systems. As an immediate mode UI, the logic behind what gets drawn to the screen in Nuklear runs every frame, and it draws/updates the entire screen every frame. A retained mode UI like QT draws elements once when they need to be updated, and they get left there until an event occurs that requires something change. So immediate mode UIs like Nuklear and Dear ImGUI, tend to be more appropriate for rendering the UI elements of games and other programs that already draw/update the screen each frame. Retained mode tends to be more useful for traditional "desktop" like applications where interacting with the standard toolkit widgets serves the primary purpose of the program. Of course there are many ways of blurring the lines and using either for both, so this isn't a hard and fast rule, just a rule of thumb.