The reason terminals and TUIs are still prevalent in tech circles is that it is a fairly good lowest common denominator. It makes sense to implement interfaces as TUIs. This then creeps into "higher-order functions" such as the terminal integrations for GitHub, or these new TUIs for e-mail. TUIs are basically 100% portable, there is no "does this work in Firefox and Safari?" worries, and the upfront development cost is lower because there are no inherent network transfers as in HTML/JS.