Kate isn't a particularly good editor and it's scripting language is quite lacking.

The reasons both Emacs and Vim have a Qt version as well instead of purely relying on a terminal protocol are obvious:

- ability to have fonts of different sizes

- ability to listen to more complicated keyboard shortcuts

- cleaner ways of opening popups

- ability to combine multiple fonts

- support for more colors

The downside of such a more complex protocol is that it's more involved to tunnel it over a network and doing attaching to X11 is also quite a bit more complicated than doing it to a terminal program.