Because I (apparently incorrectly) assumed a lot of people would appreciate the fact the mouse works so well.

Also, because getting the mouse to work well (across Windows, Mac, Linux, etc...) is freaking hard and the Terminal.Gui team is proud of their work ;-).

As a user of Terminal.Gui I appreciate that there is also solid mouse support. It serves my particular application well.

Maybe it's a lot harder to show flow through the UI with just keyboard entry. The mouse pointer trailing through the frame animates how the user's attention is moving through the UI.