One of the biggest upsides of TUIs over GUIs is that I can run any number of TUI instances. Meanwhile a GUI's developer has to decide to grace me with the ability to even open more than one window. "A tabbed interface will be sufficient!" -- yay, I'll never be able to view two screens of info at the same time.
Pretty much all GUI frameworks I have worked with behave the same, and you have to actively create singleton behavior.
Then you have Mac OS which makes it hard two create multiple processes of a GUI app. Other OSs don't necessarily behave the same.