There isn't much obvious benefit in using TUIs, but writing TUIs has a big upside: it's easier in most ways. It's trivially cross-platform, you don't have to make pixel-perfect GUIs (because it's impossible), you don't need icons or graphics (because you can't show graphics), etc.
That's the main reason. TUI programs can be created quickly, and they're mostly developer-oriented, so they just need to be useful enough and don't need to be optimized as much as a GUI program.