Mostly for practical reasons. I can't imagine how the UI would work on a phone -- either the text would be microscopic, or you'd have to scroll around constantly.
It's of course far easier to armchair design, but I could imagine it working well enough?
Each individual cell is ~12 lines of very short line lengths, yeah? You can easily fit a single cell on-screen with a keyboard while editing (or potentially while running), and then you could imagine a more abstract representation of a whole running system if you choose to zoom out while the system is running.
A few years ago I built a prototype of an iOS editor for Core War (https://en.wikipedia.org/wiki/Core_War), and it felt super-natural to have a custom keyboard whose keycaps were individual opcodes rather than ASCII characters.
Mostly for practical reasons. I can't imagine how the UI would work on a phone -- either the text would be microscopic, or you'd have to scroll around constantly.