I think it’s combination of media consumption focus, responsiveness and super light to hold for reading or writing.
When I bought it, I was going to use it just for reading and drawing. But it was with me all the time so I bought keyboard too and started ssh into my virtual machines for light work. Eventually, I was using it for programming more than drawing or reading.
And even with the iPad, there is always this friction when context switching from keyboard to pencil. I either use it in tablet mode (touch and pencil) or writing mode (and I always want some pointing device then).
If only I could find a graphic tablet similar to the apple trackpad!
It’s different environments too. If I am at my desk with external screens, I want a trackball. I don’t want to reach for the screen. But on the couch, or on the go, I wish I had a touch screen Mac laptop.